Discrete Math Seminars
Most Fridays in K9509  11:30am to 12:30pm
Please join us for this weekly seminar on a wide variety of topics under the umbrella of discrete mathematics. We gratefully acknowledge the Pacific Institute of Mathematical Sciences for their generous funding since 2010.
Watch for upcoming Seminars listed below, offered in SCK 9509 and/or on Zoom. We welcome remote participation from groups at other universities.
All talks will be announced on this page, on the department calendar (found at the bottom of the Math homepage), and on the DM Seminar mailing list. If you have an SFU email address, you can subscribe or unsubscribe to the mailing list by logging in to maillist.sfu.ca and searching for the mathdmsem list. Otherwise, you can ask any discrete math faculty member to manually add you to the list. With the permission of the speaker, we will record the talk video and upload it to the SFU Discrete Math Seminar Youtube Channel.
Current organizer: Bojan Mohar (mohar@sfu.ca) & Jesse Campion Loth
2023 Talks
Spring 2023
11 Jan 
Jesse Campion Loth, SFU  Permutations, probability and graph embeddings 
Products of permutations are used to model problems in fields ranging from representation theory and algebraic geometry to topological graph theory. I will show how probability can be used to study such problems. I will start with some algebraic results on products of symmetric group conjugacy classes. I will then show how these results can be used to solve natural topological problems. The main focus of the talk will then be using these random techniques to study how graphs may be embedded on different surfaces.  
18 Jan 
Shuxing Li, SFU  Packings of Partial Difference Sets 
As the underlying configuration behind many elegant finite structures, partial difference sets have been intensively studied in design theory, finite geometry, coding theory, and graph theory. In the first part of this talk, I will showcase that partial difference sets are indeed the fundamental structures explaining a series of “twovalued phenomena” in various mathematical branches. The second part concerns the construction of partial difference sets. Over the past three decades, there have been numerous constructions using very dif ferent and delicate approaches. Surprisingly, we discovered a common frame work unifying and extending many previous constructions. This is joint work with Jonathan Jedwab.  
2 Feb  Prof. Dimitri Leemans, Université Libre de Bruxelles  The number of string Cgroups of high rank 
(Joint with Operations Research seminar) Abstract polytopes are a combinatorial generalisation of classical objects that were already studied by the greeks. They consist in posets satisfying some extra axioms. Their rank is roughly speaking the number of layers the poset has. When they have the highest level of symmetry (namely the automorphism group has one orbit on the set of maximal chains), they are called regular. One can then use string Cgroups to study them. Indeed, string Cgroups are in onetoone correspondence with abstract regular polytopes. They are also smooth quotients of Coxeter groups. They consist in a pair $(G,S)$ where $G$ is a group and $S$ is a set of generating involutions satisfying a string property and an intersection property. The cardinality of the set $S$ is the rank of the string Cgroup. It corresponds to the rank of the associated polytope. We have just proven in the last months that if $n$ is large enough, up to isomorphism and duality, the number of string Cgroups of rank $r$ for $S_n$ (with $rgeq (n+3)/2$) is the same as the number of string Cgroups of rank $r+1$ for $S_{n+1}$. This result and the tools used in its proof, in particular the rank and degree extension, imply that if one knows the string Cgroups of rank $(n+3)/2$ for $S_n$ with $n$ odd, one can construct from them all string Cgroups of rank $(n+3)/2+k$ for $S_{n+k}$ for any positive integer $k$. The classification of the string Cgroups of rank $rgeq (n+3)/2$ for $S_n$ is thus reduced to classifying string Cgroups of rank $r$ for $S_{2r3}$. A consequence of this result is the complete classification of all string Cgroups of $S_n$ with rank $nkappa$ for $kappain{1,ldots,6}$, when $ngeq 2kappa+3$, which extends previous known results. The number of string Cgroups of rank $nkappa$, with $ngeq 2kappa +3$, of this classification gives the following sequence of integers indexed by $kappa$ and starting at $kappa = 1$. $$Sigma{kappa}=(1,1,7,9,35,48).$$ Joint work with Peter J. Cameron (University of St Andrews) and Maria Elisa Fernandes (University of Aveiro) 

8 Feb  Curtis Bright, University of Windsor  SAT Solvers, Isomorphfree Generation, and the Quest for the Minimum Kochen–Specker System 
I will describe a new approach for exhaustively generating combinatorial objects by combining a satisfiability (SAT) solver with an isomorphfree exhaustive generation method such as orderly generation. The SAT solver is able to limit the search to objects that satisfy given criteria, while the isomorphfree generation method ensures that the objects are generated up to isomorphism. The combined search procedure performs ordersofmagnitude faster than a pure SAT or pure computer algebraic approach, as the SAT solver tailors the search to the object in question while the isomorphfree generation avoids duplication of work when the search space is highly symmetrical. As a motivating example, I will discuss how this approach can be applied to search for KochenSpecker (KS) systems, an important combinatorial object arising in quantum physics. For example, a KS system is at the heart of the "KS Theorem" which intuitively states that a quantum observation cannot be understood as revealing a preexisting value, and the "Free Will Theorem" which intuitively states that if humans have free will then so must quantum particles. Since 1990, the smallest known KS system in three dimensions has contained 31 vectors and much effort has been expended trying to find a KS system of smaller size. An exhaustive computer search in 2016 was able to disprove the existence of a KS system of 21 or fewer vectors. Our SAT and orderly generation approach is over 25,000 times faster than the previously used approach and has also ruled out the existence of a KS system with 22 vectors. This work is joint with Zhengyu Li and Vijay Ganesh (University of Waterloo). 

15 Feb  Stephen Kobourov, University of Arizona  Contact Representation of Planar Graphs in 2D and 3D 
In a proportional contact representation of a planar graph, each vertex is represented by a simple polygon with area proportional to a given weight, and edges are represented by adjacencies between the corresponding pairs of polygons. We show how to use Schnyder realizers and canonical orders for planar graphs to obtain different types of contact representations. Specifically, we describe an algorithm that constructs proportional contact representation for arbitrary planar graphs using 10sided rectilinear polygons. We also describe a construction with 8sided polygons, which is optimal in terms of polygonal complexity, as 8sided polygons are sometimes necessary. In 3D vertices are represented by polytopes and edges by contacts between the corresponding polytopes contacts. We show that planar 3trees have contact representations with cubes and proportional contact representations with boxes. 

8 Mar  Harmony Zhan, SFU  On trees with extremal second largest eigenvalues 
We will characterize members that maximize/minimize the second largest adjacency eigenvalue in certain families of trees. Joint work with H. Kumar, B. Mohar and S. Pragada.  
15 Mar  Zhilin Ge, SFU  Hamiltonicity of Covering Graphs of Trees 
We consider covering graphs obtained by lifting trees as voltage graphs over cyclic groups. We generalize a tool of Hell, Nishiyama, and Stacho, known as the billiard strategy, for constructing Hamiltonian cycles in the covering graphs of paths. The presentation will show that our extended tool can be used to provide new sufficient conditions for the Hamiltonicity of covering graphs of trees that are similar to those of Batagelj and Pisanski and of Hell, Nishiyama, and Stacho. Next, we focus specifically on covering graphs obtained from trees lifted as voltage graphs over cyclic groups Zp of large prime order p. The presentation will show that for a given reflexive tree T with random nonzero voltage labels from Zp on its edges, the corresponding lift is almost surely Hamiltonian for a large enough primeordered cyclic group Zp. Finally, we show that if a reflexive tree T is lifted over a group Zp of a large prime order, then for any assignment of nonzero elements of Zp to the edges of T, the corresponding cover of T has a large circumference.  
5 Apr  Alexandra Wesolek, SFU 
Cops and Robber Game on Surfaces of Constant Curvature 
Recently, Mohar introduced a variant of the cops and robber game that is played on geodesic spaces. In this talk we discuss the rules of the game and strategies for the players when the game is played on compact surfaces of constant curvature, in particular surfaces of negative constant curvature. Joint work with Vesna Iršič and Bojan Mohar.  
14 Apr  Daniel Panario, Carleton University  The Dynamics of Iterating Functions over Finite Fields 
When we iterate functions over finite structures, there is an underlying natural functional graph. For a function $f$ over a finite field $\mathbb{F}_q$, this graph has $q$ nodes and a directed edge from vertex $a$ to vertex $b$ if and only if $f(a)=b$. It is well known, combinatorially, that functional graphs are sets of connected components, components are directed cycles of nodes, and each of these nodes is the root of a directed tree. The study of iterations of functions over a finite field and their corresponding functional graphs is a growing area ofresearch, in part due to their applications in cryptography and integer factorization methods like Pollard rho algorithm. Some functions over finite fields when iterated present strong symmetry properties. These symmetries allow mathematical proofs of some dynamical properties such as period and preperiod of a generic element, (average) ``rho length'' (number of iterations until a cycle is formed), number of connected components, cycle lengths, and permutational properties (including the cycle decomposition). We survey the main problems addressed in this area so far. We briefly comment on ongoing research about the functional graph of generalized cyclotomic mappings of finite fields. We study periodic points, cycle structure, and rooted trees attached to periodic points. We briefly comment on some open problems. Based on joint works with Alexander Bors (Carleton, Canada), Rodrigo Martins (UTFPR, Brazil), Claudio Qureshi (UdelaR, Uruguay), Lucas Reis (UFMG, Brazil) and Qiang (Steven) Wang (Carleton, Canada). 
2022 Talks
Fall 2022
07 Oct Special Time: 11am 
Bojan Mohar. Simon Fraser University  Beyond the FourColor Theorem 
The speaker will discuss the FourColor Theorem and some of the outstanding problems left open after the proof.  
14 Oct 
Peter Bradshaw, Simon Fraser University  A small step forward in bipartite list coloring 
The choosability of a graph G is the minimum value k such that G has a proper list coloring whenever each vertex of G has a list of k colors. Molloy showed that if G is a trianglefree graph of maximum degree d, then the choosability of G is at most (1 + o(1)) d / log d and that this bound is tight within a factor of roughly 2. It is widely believed that this upper bound can be greatly improved in the special case that G is bipartite, and Alon and Krivelevich conjectured an upper bound of O(log d) in this case. However, little progress has been made toward solving this conjecture, and it is currently unknown whether the optimal upper bound for bipartite graphs is different from the optimal upper bound for trianglefree graphs. In this talk, we will use a modification of a couponcollector argument from Alon, Cambie, and Kang to show that every bipartite graph of maximum degree d has choosability at most 0.797 d / log d. Our result provides strong evidence that the listcoloring problem is fundamentally easier in bipartite graphs than in trianglefree graphs and hence takes an important step toward solving the conjecture of Alon and Krivelevich. (The content of this talk is part of ongoing work with Bojan Mohar and Ladislav Stacho.) 

21 Oct  Harmony Zhan, Simon Fraser University  An introduction to discrete quantum walks 
A discrete quantum walk is determined by a unitary matrix representation of a graph. In this talk, I will give an overview of different quantum walks, and show how the spectral information of the unitary matrix representation links properties of the walks to properties of the graphs. Part of this talk will be based on the book, Discrete Quantum Walks on Graphs and Digraphs, by Chris Godsil and me.  
28 Oct  Zhouningxin Wang  Circular flows in monodirected Eulerian signed graphs 
In this talk, we consider analogs of Jaeger's circular flow conjecture and its dual JaegerZhang conjecture in signed graphs. We will first give the notions of circular coloring and circular flow in signed graphs, and then show that every (6k2)edgeconnected Eulerian signed graph admits a circular 4k/(2k1)flow and that every signed bipartite planar graph of negativegirth at least 6k2 admits a circular 4k/(2k1)coloring (equivalently, admits a homomorphism to a negative cycle of length 2k). We also provide some recent results about circular flow indices of signed graphs with high edgeconnectivities. This is based on joint work with Jiaao Li, Reza Naserasr, and Xuding Zhu. 

4 Nov  Dong Ye, Middle Tennessee State University  Nowherezero Z3flow and signcircuit covering 
In 1950s, Tutte discovered that the mapcoloring problem on orientable surface could be interpreted as an integer flow problem. A classic result of Tutte states that a graph with a nowherezero Z3flow has a circuit cover which covers every edge at most twice, and hence has a shortest circuit cover with length at most 4/3 of the total number of edges. For an orientable graphs on nonorientable surfaces, its dual is a signed graph. In this talk, we focus on nowherezero Z3flows and circuit covers of signed graphs, extending Tutte’s result from graph to signed graphs. This is based on joint work with Jiaao Li and Yezhou Wu.  
18 Nov  Daniel Neuen, SFU  Parameterized Algorithms for Graph Isomorphism  Decompositions via Regularity 
In the first part of the talk, I will give an overview of recent advances on the graph isomorphism problem focusing on quasipolynomial parameterized algorithms with a running time of the form n^{polylog(k)}, where k is some graph parameter. The first such algorithm was obtained for k being the maximum degree of the input graphs [FOCS 2018] by extending the grouptheoretic tools of Babai's quasipolynomial time isomorphism test [STOC 2016]. Subsequently, this result was generalized in a series of works to all graphs that exclude the complete graph on k vertices as a topological subgraph [SODA 2022]. In particular, this includes all graphs of Euler genus at most k7 as well as all graphs of treewidth at most k2. The key concept underlying this latest algorithm is the notion of tCRbounded graphs that serves as a gateway to combine the grouptheoretic tools underlying the isomorphism test for boundeddegree graphs with decompositionbased approaches for graphs that exclude the complete graph on k vertices as a topological subgraph. In the second part of the talk, we explore the graphtheoretic side of the algorithm in more depth and discuss how regularity of the input graphs can be leveraged to obtain decompositions into parts with few symmetries that can then be handled by the grouptheoretic graph isomorphism machinery.  
25 Nov  Jonathan Jedwab, SFU  Constructions of difference sets in groups of order $4^d$ 
The study of difference sets lies at the intersection of design theory, coding theory, finite geometry, and algebraic number theory. The central problem is to determine which groups contain at least one difference set, and the richest existence results occur when the group order is a power of 4. The principal obstacles are that the number of such groups grows rapidly, and that the groups have such diverse structures that an overall theory seems out of reach. However, after a tenyear collaboration, this question was resolved completely for all 56,092 groups of order 256 using a new theoretical framework together with computational search. I shall describe the history of this problem, the methods used to resolve order 256, and the prospects for extending these techniques to order 1024. 
Spring 2022
18 Jan (ZOOM)  Jiaxi Nie, University of California San Diego  On Asymptotic Packing of Geometric Graphs 
A set of geometric graphs is geometricpackable if it can be asymptotically packed into every sequence of drawings of the complete graph $K_n$. For example, the set of geometric triangles is geometricpackable due to the existence of Steiner Triple Systems. When G is the 4cycle (or 4cycle with a chord), we show that the set of plane drawings of G is geometricpackable. In contrast, the analogous statement is false when G is nearly any other planar Hamiltonian graph (with at most 3 possible exceptions). A convex geometric graph is convexpackable if it can be asymptotically packed into the convex drawings of the complete graphs. For each planar Hamiltonian graph G, we determine whether or not a plane G is convexpackable. Many of our proofs explicitly construct these packings; in these cases, the packings exhibit a symmetry that mirrors the vertex transitivity of $K_n$. here is the link. (ID: 668 0051 2140, password: Graph) 

25 Jan  Kathryn Nurse, Simon Fraser University  Toward Bouchet’s conjecture on nowherezero flows in signed graphs. 
I will present a work in progress. In 1954, Tutte proved flowcolouring duality and made a conjecture that every graph without a cutedge has a nowherezero 5flow. A parallel conjecture to this, but for signed graphs is Bouchet's conjecture (1983) that every signed graph without the obvious obstruction has a nowherezero 6flow. We have been able to show that Bouchet's conjecture holds for 3connected graphs when 6 is replaced with 8. This is joint with Matt DeVos and Robert Šámal.  
01 Feb  Annie Raymond (UMass)  Tropicalization of graph profiles 
The number of homomorphisms from a graph H to a graph G, denoted by hom(H;G), is the number of maps from V(H) to V(G) that yield a graph homomorphism, i.e., that map every edge of H to an edge of G. Given a fixed collection of finite simple graphs {H_1, ..., H_s}, the graph profile is the set of all vectors (hom(H_1; G), ..., hom(H_s; G)) as G varies over all graphs. Graph profiles essentially allow us to understand all polynomial inequalities in homomorphism numbers that are valid on all graphs. Profiles can be extremely complicated; for instance the full profile of any triple of connected graphs is not known. To simplify these objects, we introduce their tropicalization which we show is a closed convex cone that still captures interesting combinatorial information. We explicitly compute these tropicalizations for some sets of graphs, and relate the results to some questions in extremal graph theory. This is joint work with Greg Blekherman, Mohit Singh and Rekha Thomas.  
08 Feb  Vesna Iršič (Simon Fraser University)  Winning vs. catching in the game of cops and robber on manifolds 
Recently, Mohar introduced a variant of the cops and robber game that is played on geodesic spaces. The game combines properties of pursuitevasion games with the classical cops and robber game played on graphs. In the game, cops win if they can get arbitrarily close to the robber. On the other hand, cops catch the robber if one of them occupies the same point as the robber. In this talk we will discuss several strategies for players in the game, and observe the difference between the number of cops needed to catch the robber and the number of cops needed to win the game. Joint work with Bojan Mohar and Alexandra Wesolek.  
15 Mar  Bojan Mohar (Simon Fraser University)  Proper orientations and proper chromatic number 
It is an interesting (and not entirely obvious) fact that every graph admits an orientation of its edges so that the outdegrees at vertices form a "coloring" (outdegrees of any two adjacent vertices are different). The proper chromatic number $\Vec{\chi}(G)$ of a graph $G$ is the minimum $k$ such that there exists an orientation of the edges of $G$ with all vertexoutdegrees at most $k$ and such that for any adjacent vertices, the outdegrees are different. Two major conjectures about the proper chromatic number are resolved. First it is shown, that $\Vec{\chi}(G)$ of any planar graph $G$ is bounded (in fact, it is at most 14). Secondly, it is shown that for every graph, $\Vec{\chi}(G)$ is at most $O(\frac{r\log r}{\log\log r})+\tfrac{1}{2}\MAD(G)$, where $r=\chi(G)$ is the usual chromatic number of the graph, and $\MAD(G)$ is the maximum average degree taken over all subgraphs of $G$. Several other related results are derived. Our proofs are based on a novel notion of fractional orientations. This is joint work with Yaobin Chen and Hehui Wu. 

05 Apr  Zdenek Dvorak (Charles University)  3coloring nearquadrangulations of the plane and the torus using flows (joint work with C. Bang, E. Heath, and B. Lidicky) 
A graph drawn in a surface is a knearquadrangulation if the product of the lengths of its faces, except for 4faces, is at most k. Nearquadrangulations play a fundamental role in the theory of 3colorability of trianglefree graphs on surfaces. We show how the wellknown duality between colorings and flows can be used to design * a 3precoloring extension algorithm for knearquadrangulations of the plane, and * a 3coloring algorithm for knearquadrangulations of the torus, with time complexity polynomial in k and the number of vertices. 
2021 Talks
Fall 2021
09 Nov 
Andrei Bulatov, Simon Fraser University 
The complexity of counting homomorphisms 
The Constraint Satisfaction Problem (CSP) provides a general framework for a wide range of combinatorial problems. The simplest way to state the problem is: Given relational structures (say, graphs) G and H, decide if there is a homomorphism from G to H. The counting version of the CSP, #CSP, asks for the number of such homomorphisms. In order to represent specific problems, the CSP is often restricted in various ways, in particular by fixing the target structure H. Thus, CSP(H) asks if, for a given structure G, there is a homomorphism from G to H. If H is a graph such a problem is often called HColoring. The counting versions #CSP(H), #HColoring of these problems are defined accordingly. In this talk we survey the developments in the study of the complexity of #CSP(H) that eventually led to a complete classification of such problems and their various generalizations. ZOOM Information: https://sfu.zoom.us/j/66800512140?pwd=N011dU5ZNThSWnVzMFBjYnBidDdmUT09 (ID: 668 0051 2140 Password: Graph) 

16 Nov  Peter Bradshaw, Simon Fraser University  A rainbow connectivity threshold for random graph families 
Given a family G of graphs on a common vertex set X, we say that G is rainbow connected if for every vertex pair u, v ∈ X, there exists a path from u to v that uses at most one edge from each graph of G. We consider the case that G contains s graphs, each sampled randomly from G(n, p), with n = X and p = c log n / sn , where c > 1 is a constant. We show that there exists a threshold of at most three consecutive integer values such that when s is greater than or equal to this threshold, G is a.a.s. rainbow connected, and when s is below this threshold, G is a.a.s. not rainbow connected. Joint work with Bojan Mohar. 
Summer 2021
19 May 
Joy Morris, University of Lethbridge 
Lexicographic products and wreath products 
Abstract: I will present a history and overview of some of the work that has been done on the lexicographic product of graphs, and related generalisations. The focus of my talk will be on the automorphism groups of such graphs, and the relationship to the wreath product of permutation groups.  
2 June 
Xiangdong Hou, University of South Florida 
On the number of equivalence classes of boolean functions 
Abstract: Two Boolean functions from $\Bbb F_2^n$ to $\Bbb F_2$ are called (affine) equivalent if one can be obtained from the other through an invertible affine transformation of the variables followed by an addition of an affine function. Most coding theoretic and cryptographic properties of Boolean functions are preserved under this equivalence. Let $N_n$ denote the number of equivalence classes of Boolean functions in $n$ variable. No explicit formula for $N_n$ is known. A longstanding open question by MacWilliams and Sloane asks for an asymptotic formula for $N_n$ as $n\to\infty$. Recently, we find a solution to this question. (Abstract in PDF)  
8 June*Tuesday @ 2 pm* 
Special session for the Round the World Relay in CombinatoricsFan Chung, University of California San Diego 
Trees and forests in Green's functions of a graph 
Abstract: The Green's functions of a graph are the pseudo inverses of the Laplacians and therefore are useful for solving many types of Laplace equations in discrete settings.In this talk, we will give combinatorial interpretations of Green's functions in terms of counting trees and forests in a graph. We will also mention several applications concerning the pagerank algorithms and the hitting time for random walks. 

16 June 
Akbar Rafiey, SFU 
Fast and Private Submodular and kSubmodular Functions Maximization with Matroid Constraints 
Abstract: The problem of maximizing nonnegative monotone submodular functions under a certain constraint has been intensively studied in the last decade, and a wide range of efficient approximation algorithms have been developed for this problem. Many machine learning problems, including data summarization and influence maximization, can be naturally modeled as the problem of maximizing monotone submodular functions. However, when such applications involve sensitive data about individuals, their privacy concerns should be addressed. In this paper, we study the problem of maximizing monotone submodular functions subject to matroid constraints in the framework of differential privacy. We provide (1−1/e)approximation algorithm which improves upon the previous results in terms of approximation guarantee. This is done with an almost cubic number of function evaluations in our algorithm. Moreover, we study ksubmodularity, a natural generalization of submodularity. We give the first 1/2approximation algorithm that preserves differential privacy for maximizing monotone ksubmodular functions subject to matroid constraints. The approximation ratio is asymptotically tight and is obtained with an almost linear number of function evaluations. Joint work with Yuichi Yoshida (http://proceedings.mlr.press/v119/rafiey20a.html) 

23 June

Rebecca Patrias, University of St Thomas 
Webs and tableau promotion 
Abstract: We will start with an introduction to webs, standard Young tableau promotion, and the relationship between web rotation and tableau promotion. We will then discuss increasing tableauxa Ktheoretic analogue of SYTand Kpromotion. A big open question in this area deals with the order of Kpromotion on increasing tableaux. We will talk about results in the 3row case (in joint work with O. Pechenik) as well as current efforts to relate this problem to web promotion (joint with O. Pechenik, J. Striker, and J. Tymoczko).  
30 June 
Haggai Liu 
Enumerative Properties of Cogrowth Series on Free products of Finite Groups 
Abstract: Given a group G with a finite set of generators, S, it is natural to ask if the product of n generators from S evaluate to the identity. The enumerative version of this problem, known as the cogrowth problem, counts the number of such products and studies the associated counting sequence. Many cogrowth sequences are known. We focus on the free products of finite groups: Specifically, cyclic and dihedral groups. Such groups have the property that their cogrowth generating functions are algebraic functions, and thus, are solutions to implicit polynomial equations. Using algebraic elimination techniquesand free probability theory, we establish upper bounds on the degrees of the polynomial equations that they satisfy. This has implications for asymptotic enumeration, and makes it theoretically possible to determine the functions explicitly.  
7 July 
Christin Bibby, Louisiana State University 
Combinatorics of orbit configuration spaces 
Abstract: From a group action on a space, define a variant of the configuration space by insisting that no two points inhabit the same orbit. When the action is almost free, this "orbit configuration space" is the complement of an arrangement of subvarieties inside the cartesian product, and we use this structure to study its topology. We give an abstract combinatorial description of its poset of layers (connected components of intersections from the arrangement) which turns out to be of much independent interest as a generalization of partition and Dowling lattices. The close relationship to these classical posets is then exploited to give explicit cohomological calculations.  
14 July 
Yifan Jing, UIUC 
Minimal and nearly minimal measure expansions in locally compact groups 
Abstract: In 1964, Kemperman proved the following continuous nonabelian counterpart of the CauchyDavenport theorem: If G is a connected unimodular locally compact group with a left (and hence right) Haar measure \mu, A, B \subseteq G are nonempty and compact, and AB is their product set, then \mu(AB) \geq \min\{ \mu(A) + \mu(B), \mu(G)\}. I will present the recent joint works with Jinpeng An, ChieuMinh Tran and Ruixiang Zhang where we determine the conditions for the equality to happen or nearly happen in the above inequality. Our results and methods answer several questions by Griesmer, Henstock, Hrushovski, Kemperman, Macbeath, McCrudden, and Tao. 

21 July 
Sophie Spirkl, Waterloo 
The complexity of list5colouring with a forbidden induced sugbraph 
Abstract: A klistassignment for a graph G is a function L from V(G) to the set of subsets of {1,…,k}. The listkcolouring problem asks, given G and a klistassignment L, is there a colouring f of G with f(v) in L(v) for all v in V(G)? This generalizes the kcolouring problem (where we use L(v)={1,…,k} for every vertex). For k at least 3, both kcolouring and listkcolouring are NPhard, which motivates studying the complexity of these problems when the input is restricted to Hfree graphs (for a fixed H, excluded as an induced subgraph). I will present an algorithm for list5colouring restricted to Hfree graphs for all H which consist of connected components each of which is a 3vertex path. This, together with previous results, gives a complete answer to the question, “for which H is list5colouring restricted to Hfree graphs NPhard? Joint work with Sepehr Hajebi and Yanjia Li. 

28 July 
Anurag Bishnoi, TU Delft 
The minimum degree of minimal Ramsey graphs for cliques 
Abstract: In this talk we will present a new upper bound of $s_r(K_t) = O(t^5r^{5/2})$ on the Ramsey parameter $s_r(K_t)$ introduced by Burr, Erd\H{o}s and Lov\'{a}sz in 1976, which is defined as the smallest minimum degree of a graph $G$ such that any $rcolouring of the edges of $G$ contains a monochromatic $K_t$, whereas no proper subgraph of $G$ has this property. This improves the previous upper bound of $s_r(K_t) = O(t^6r^3)$ proved by Fox et al. The construction used in our proof relies on a group theoretic model of generalised quadrangles introduced by Kantor in 1980. This is joint work with John Bamberg and Thomas Lesgourgues (https://arxiv.org/abs/2008.02474). 
Spring 2021
20 January

Peter Bradshaw, SFU 
Flexible list colorings in graphs of bounded degree 
^{Abstract: For a given $\epsilon > 0$, we say that a graph $G$ is $\epsilon$flexibly $k$choosable if the following holds: for any assignment $L$ of lists of size $k$ on $V(G)$, if a preferred color is requested at any set $R$ of vertices, then at least $\epsilon R$ of these requests are satisfied by some $L$coloring. We characterize the graphs of maximum degree $\Delta$ that are $\epsilon$flexibly $\Delta$choosable for some $\epsilon = \epsilon(\Delta) > 0$, which answers a question of Dvořák, Norin, and Postle [List coloring with requests, JGT 2019]. In particular, we show that for any $\Delta\geq 3$, any graph of maximum degree $\Delta$ that is not isomorphic to $K_{\Delta+1}$ is $\frac{1}{2 \Delta^3}$flexibly $\Delta$choosable. This is joint work with Ladislav Stacho and Tomáš Masařík.}  
27 January

Ambat Vijayakumar, Cochin University of Science and Technology, India 
Distance spectrum of graphs  some recent studies 
^{Abstract: The distance matrix of a connected graph G is an n × n matrix $D(G) = [d_{ij}]$, where $d_{ij}$ is the distance between the vertices $v_i$ and $v_j$ . The distance spectrum of G is ${\lambda_1, . . . , \lambda_n}$, where $\lambda_i$'s are the eigenvalues of D(G). The distance energy of G is defined by $E_D(G) = \sum_{i=1}^n \lambda_i$. The distance matrix, dstance eigenvalue, and distance energy of a connected graph have been studied intensively in literature. We discuss a new problem of how the distance energy changes when an edge is deleted. Similar problem for adjacency energy of a graph was studied by Day and So. It turns out that the results for distance energy change and adjacency energy change are quite different. It follows that, for any connected graph with a unique positive eigenvalue, the deletion of any edge increases the distance energy provided that the resulting graph is still connected. We prove that the distance energy of a complete bipartite graph is always increased when an edge is deleted even though it has two positive distance eigenvalues. Also, we give a set of examples of connected graph whose distance energy decreases when a specific edge is deleted. This is a joint work with G. Indulal and A. Varghese.}  
3 February 
Damanvir Binner, SFU 
Proofs of Berkovich and Uncu conjectures regarding partition inequalities 
^{Abstract: We use techniques from elementary number theory (such as Frobenius Numbers) to combinatorially prove four recent conjectures of Berkovich and Uncu (Ann. Comb. 23 (2019) 263–284) regarding inequalities between the sizes of two closely related sets consisting of integer partitions whose parts lie in the interval {s, . . . , L+s}. Further restrictions are placed on the sets by specifying impermissible parts as well as a minimum part. (Joint work with A. Rattan) }  
10 February 
Pengyu Liu, SFU 
Polynomial treebased analysis 
^{Abstract: We define a graph polynomial for unlabeled trees and show that the polynomial is a complete isomorphism invariant for unlabeled trees. We also demonstrate some applications of the tree distinguishing polynomial in treebased analysis with examples in life sciences and linguistics.}  
17 February Reading Break 

^{ }  
24 February

Harmony Zhan, York University 
Arcreversal quantum walks 
^{Abstract: A discrete quantum walk usually takes place on the arcs of a graph. Each step of the walk consists of two operations  a "coin flip" and an "arc shift". In this talk, we will consider quantum walks where the "arc shift" simply reverses each arc. Using different coins, I will show the connection of these walks to various graph structures, such as line digraphs, covers and embeddings. }  
2 March*TuesDAY @ 11:3013:30* 
SpeCial Session: Nancy Ann Neudauer, Pacific University 
Being Together and Better: Building, Belonging,and Giving Back to Mathematical Communities 
^{Abstract: What have I learned about building mathematical communities over the last twenty years, initially in North America and more recently in Africa and South America? What does “belonging” mean to those who are already part of a mathematical community, and how can we provide the sort of welcome we once received (or would have liked to receive)? What constructive actions can we take to help mathematics students transition to a researchbased academic career, and how might these differ when the students are located in developing countries without access to established infrastructure? Dr. Neudauer will discuss these and other questions in a special session of the SFU Discrete Mathematics Seminar, intended for students and faculty at all career stages. She will share stories and insights gained from decades of service to the mathematical community as a researcher, teacher, mentor, and organizer.}  
10 March 
Zeying Wang, Michigan Tech 
Partial difference sets in abelian groups 
^{Abstract: Recently we proved a theorem for strongly regular graphs that provides numerical restrictions on the number of fixed vertices and the number of vertices mapped to adjacent vertices under an automorphism. We then used this result to develop some new techniques to study regular partial difference sets in abelian groups. We have proved several nonexistence results and classification results of partial difference sets in abelian groups. Also we completely answered the question "For which odd positive integer $v>1$, can we find a Paley type partial difference set in an abelian group of order $v$?", a classical question from the 1990s. In this talk I plan to give an overview of these results.}  
17 March 
Riana Roux, Stellenbosch University 
A localization game on graphs 
^{Abstract: The classic game of cops and robbers on a graph is a pursuitevasion game introduced in the late 1970's. In the original game the cops and robber take turns to either stay at their present vertex or move to an adjacent vertex. The cops win the game if at any moment a cop occupies the same vertex as the robber. In this talk we will consider a game where the robber is invisible and where the cop only has to locate the robber, that is, the cop does not have to be on the same vertex as the robber to win. This game is called the localization game and consist of two players: a team of k cops and a robber. The game starts with a robber occupying some vertex on the underlying graph. The cop team then probes k vertices and receives the distance of the robber to each of these probed vertices. If the cop can deduce the robber’s location from this information he wins, otherwise the robber may move to an adjacent vertex and the cop again probes k vertices. If the cop can locate the robber in a finite number of turns, the cop wins. Otherwise, the robber wins. The localization number of a graph is the smallest k for which the cop wins. During this talk we will discuss some basic results regarding the localization number, ideas around forbidden subgraphs and the game played on specific graphs classes.}  
24 March 
Daniel Panario, Carleton University 
Finite fields in combinatorial arrays: constructions and applications 
^{Abstract: We discuss constructions based on finite fields of three types of combinatorial arrays: orthogonal, covering and ordered orthogonal arrays. Other combinatorial arrays have been similarly considered in the literature, including permutation arrays, frequency permutation arrays, hypercubes and Costas arrays. In this talk, for each of these arrays, we briefly explain constructions based on finite fields, and main applications. An Orthogonal Array (OA) of strength $t$ on $v$ symbols is an array with the property that, for every $t$combination of column vectors, every one of the possible $v^t$ $t$tuples of symbols appears as a row ``exactly'' once in the subarray defined by these column vectors. OAs have been applied in coding theory and in cryptography. They are equivalent to MDS (maximum distance separable) codes where the strength of the OA is closely related to the minimum distance of the code. OAs are also closely related to secret sharing in cryptography. An Ordered Orthogonal Arrays (OOA) is a generalization of OAs where the coverage property applies to some selected columns of the array. Similarly, a Covering Array of strength $t$ on $v$ symbols is an array with the property that, for every $t$combination of column vectors, every one of the possible $v^t$ $t$tuples of symbols appears as a row ``at least'' once in the subarray defined by these column vectors. Covering arrays are used to reduce the number of tests in application areas such as software testing and group testing. A common theme on several recent constructions of these arrays is the use of linear feedback shift register (LFSR) sequences and finite fields. Arrays whose rows are cyclic shifts of a sequence over a finite field possess many combinatorial properties. They have been used to build arrays attaining a high number of $t$subsets of columns with the desired ``coverage property''. We provide examples of applications of these combinatorial arrays. }talk slides  
31 March 
Tomáš Masařík, SFU 
Optimal discretization is fixedparameter tractable 
^{Abstract: Given two disjoint sets W_1 and W_2 of points in the plane, the Optimal Discretization problem asks for the minimum size of a family of horizontal and vertical lines that separate W_1 from W_2, that is, in every region into which the lines partition the plane there are either only points of W_1, or only points of W_2, or the region is empty. Equivalently, Optimal Discretization can be phrased as a task of discretizing continuous variables: we would like to discretize the range of xcoordinates and the range of ycoordinates into as few segments as possible, maintaining that no pair of points from W_1×W_2 are projected onto the same pair of segments under this discretization. We provide a fixedparameter algorithm for the problem, parameterized by the number of lines in the solution. Our algorithm works in time 2^O(k^2 log(k))n^O(1), where k is the bound on the number of lines to find and n is the number of points in the input. Our result answers in positive a question of Bonnet, Giannopolous, and Lampis [IPEC 2017] and of Froese [PhD thesis, 2018] and is in contrast with the knownintractability of two closely related generalizations: the Rectangle Stabbing problem and the generalization in which the selected lines are not required to be axisparallel. Joint work with: Stefan Kratsch, Irene Muzi, Marcin Pilipczuk, Manuel Sorge}  
7 April 
Amy Wiebe, Free University of Berlin 
A combinatorial approach to Minkowski tensors of polytopes 
^{Abstract: Intrinsic volumes of a convex body provide scalar data (volume, surface area, Euler characteristic, etc. ) about the geometry of a convex body independent of the ambient space. Minkowski tensors are the tensorvalued generalization of intrinsic volumes. They provide more complex geometric information about a convex body, such as its shape, orientation, and more. Minkowski volume tensors are closely linked to the moments of the uniform distribution on a convex body, and a rational generating function for these moments allows us to extract the tensors symbolically. In this talk, we explain this connection and show that it can be extended to the setting of Minkowski "surface tensors". We demonstrate how this generating function approach allows us to give an explicit formula for these surface tensors in the case of simplicial polytopes. No prior knowledge of Minkowski tensors will be assumed. This is a joint work with Büşra Sert and Niklas Livchitz}  
14 April 
Carolyn Chun, United States Navel Academy 
Graphs and binary matroids whose odd circuits all have size three or five 
^{Abstract: Graphs with no odd circuits are well understood. In 1992, Maffray characterized all graphs whose odd circuits only have size three. Oxley and Wetzler generalized this result to binary matroids in 2016. We give a complete characterization of all binary matroids whose odd circuits all have size three or five, as well as the graphs whose odd cycles all have size three or five. This talk is based on joint work with Oxley and Wetzler. }talk slides 
2020 Talks
Fall 2020
16 September  Marthe Bonamy, LaBRI  On Vizing's edge colouring question 
In his 1965 seminal paper on edge colouring, Vizing proved that a (Delta+1)edge colouring can be reached from any given proper edge colouring through a series of Kempe changes, where Delta is the maximum degree of the graph. He concludes the paper with the following question: can an optimal edge colouring be reached from any given proper edge colouring through a series of Kempe changes? In other words, if the graph is Deltaedgecolourable, can we always reach a Deltaedgecolouring? If true, this would imply a more recent conjecture of Mohar (2006) that in any graph, all (Delta+2)edgecolourings are equivalent up to a series of Kempe changes. We discuss recent progress around these questions.  
23 September  Carla Groenland, University of Oxford  Asymptotic dimension of graph classes 
I will attempt to give some intuition for asymptotic dimension of graph classes, starting from the definition and giving the connection to clustered colouring and weakdiameter network decompositions. I will sketch the ideas behind some of our results, e.g. that the class of planar graphs has asymptotic dimension 2. Asymptotic dimension was introduced by Gromov in 1993 within geometric group theory and in the graph setting makes sense for infinite graphs or graph classes. Joint work with M. Bonamy, N. Bousquet, L. Esperet, F. Pirot, and A. Scott.  
30 September  Karen Yeats, University of Waterloo  Chains of hourglasses and the graph theory of their Feynman integrals 
I will give a set up for understanding the graph theory of Feynman integrals that I have been using over the last number of years. It is all very combinatorial and does not require any understanding of physics or analysis. Then I will explain about some joint work with Oliver Schnetz where we are able to complete a generalized denominator reduction on a particular family of graphs and from this calculate an arithmetic graph invariant on them known as the c_2 invariant as well as understand properties of their Feynman integrals.  
7 October  Shanise Walker, University of Wisconsin Eau Claire  The Game of Cycles 
The Game of Cycles was introduced by Francis Su in his book titled Mathematics for Human Flourishing (2020). The game is played on a simple connected planar graph together with its bounded cells, and players take turns marking edges with arrows according to a sinksource rule. The object of the game is to produce a cycle cell–a cell surrounded by arrows all cycling in one direction–or to make the last possible move. In this talk, we explore the twoplayer game for various classes of graphs and determine who has a winning strategy.  
14 October  Laura Escobar, Washington University in St. Louis  Which Schubert varieties are Hessenberg varieties? 
Schubert varieties are subvarieties of the flag variety whose geometry can be understood using the combinatorics of permutations. Hessenberg varieties are also subvarieties of the flag variety with connections to both algebraic combinatorics and representation theory. After introducing and motivating these varieties, I will discuss joint work with Martha Precup and John Shareshian in which we investigate which Schubert varieties in the full flag variety are Hessenberg varieties.  
21 October  Inês Rodrigues, University of Lisbon  An action of the cactus group on the shifted tableau crystal 
The shifted tableau crystal is a crystallike structure on shifted semistandard tableaux, introduced by Gillespie, Levinson and Purboo (2017). We define on this structure a shifted version of the crystal reflection operators, originally introduced by Lascoux and Schützenberger on type A crystals, which coincide with the restrictions of the shifted Schützenberger involution on primed intervals of two adjacent letters. Unlike type A crystal they not yield an action of the symmetric group. Following a similar approach as Halacheva (2016), we then exhibit a natural action of the cactus group on the shifted tableau crystal, realized by the restrictions of the shifted Schützenberger involution to all primed intervals of the underlying crystal alphabet, including, in particular, the crystal reflection operators.  
28 October  Jana Novotná, University of Warsaw/Charles University  Cliquewidth of Atoms and Coloring for Hereditary Graphs 
Cliquewidth is a wellestablished graph parameter. Many NPcomplete graph problems are polynomially solvable on graph classes of bounded cliquewidth. Moreover, several of these problems are polynomially solvable on a hereditary graph class G if they are so on the atoms (graphs with no clique cutset) of G. For these reasons, the boundedness of cliquewidth has been investigated for many graph classes. In this talk, we consider graph classes defined by one or two forbidden induced subgraphs. We discuss the coloring problem and the boundedness of cliquewidth of their atoms.  
4 November  Karen Meagher, University of Regina  Erdős–Ko–Rado Theorems for Groups 
The first half of this talk will be a gentle introduction to the Erdős–Ko–Rado (EKR) Theorem. This is a theorem that determines the size and structure of the largest collection of intersecting sets. It has become a cornerstone of extremal set theory and has been extended to many other objects. I will show how this result can be proven using techniques from algebraic graph theory. In the second half of this talk I will give more details about extensions of the EKR theorem to permutations. Two permutations are intersecting if they both map some i to the same point (so σ and π are intersecting if and only if σ^{−1}π has a fixed point). In 1977, Deza and Frankl proved that the size of a set of intersecting permutations is at most (n1)!. It wasn't until 2003 that the structure of sets of intersecting permutations that meet this bound was determined. For any group action, we can ask what is the size and structure of the largest set of intersecting permutations. I will show some recent results for different groups, with a focus on 2transitive groups. 

12 November 2:303:20pm  Patricia Hersh, University of Oregon  Posets arising as 1skeleta of simple polytopes, the nonrevisiting path conjecture, and poset topology 
Held jointly with the SFU Operations Research Seminar Note the nonstandard time and day of the week  
Abstract: Given a polytope P and a nonzero vector c, the question of which point in P has largest inner product (or smallest inner product) with c is the main goal of linear programming in operations research. Key to efficiency questions regarding linear programming is the directed graph G(P,c) on the 1skeleton of P obtained by directing each edge e(u,v) from u to v for c(u) < c(v) and in particular the diameter of G(P,c). We will explore the question of finding sufficient conditions on P and c to guarantee that no directed path ever revisits any polytope face that it has left; this is enough to ensure that linear programming is efficient under all possible choices of pivot rule. It turns out that posettheoretic techniques and poset topology can help shed light on this question. In discussing work on this question, we will provide background along the way.  
18 November  Natasha Morrison, University of Victoria  Hypergraph Lagrangians: Extremal Combinatorics and the FranklFüredi conjecture 
In this talk we explore the origins of hypergraph Lagrangians and their applications to classical problems in extremal combinatorics. We then focus on the FranklFüredi conjecture, which concerns the maximal value of the Lagrangian of an runiform hypergraph with m edges. In particular, we show that the conjecture is false in general, but holds for all sufficiently large 3uniform hypergraphs. This talk is based on joint work with Vytautas Gruslys and Shoham Letzter.  
25 November  Amélie Trotignon, Johannes Kepler University  Discrete harmonic functions in the threequarter plane 
Harmonic functions play an important role in probability theory and are strongly related to the enumeration of walks. Doob htransform is a way to build conditioned random processes in cones from a random process and a positive harmonic function vanishing on the boundary of the cone. Finding positive harmonic functions for random processes is therefore a natural objective in the study of confined random walks. There are very few ways to compute discrete harmonic functions. In this talk we are interested in positive discrete harmonic functions with Dirichlet conditions in three quadrants. Whereas planar lattice (random) walks in the quadrant have been well studied, the case of walks avoiding a quadrant has been developed lately. We extend the method in the quarter plane – resolution of a functional equation via boundary value problem using a conformal mapping – to the threequarter plane applying the strategy of splitting the domain into two symmetric convex cones. We obtain a simple explicit expression for the algebraic generating function of harmonic functions associated to random walks avoiding a quadrant.  
2 December  Sofiat Olaosebikan, University of Glashow  Algorithmics of the StudentProject Allocation problem 
Matching problems are all around us – they arise when we try to find the “best” allocation between two sets of entities. For example, matching transplant patients with organ donors, allocating junior doctors to hospitals, students to projects, and wireless networks to users. This class of problems was first studied in 1962 by Gale and Shapley, and its founding researchers were awarded a Nobel prize in 2012. My talk is centred around the StudentProject Allocation problem (SPA), which, in its natural form, involves finding a manytoone matching of students to projects offered by lecturers, based on student preferences over projects and the maximum number of students that each project and lecturer can accommodate. During my PhD, I explored SPA with lecturer preferences over Projects (SPAP), SPA with lecturer preferences over Students (SPAS), and SPAS with Ties (SPAST). The solution concept that we seek in this context is a “stable matching” – a matching that ensures that no student and lecturer who are not paired together would rather be assigned to each other than remain with their current assignees. In my talk, I will present the structural and algorithmic results arising from my research on finding stable matchings in the problem models mentioned above.  
8 December  Aparna Lakshmanan, St. Xavier's College  On Leech Labeling 
Note the nonstandard day of the week  
A tree is called a Leech tree, if one can assign positive integer edge weights such that the nC2 path weights are exactly 1, 2,..., nC2, where the weight of a path p, w(P) is the sum of weights of all its edges and such a labeling is called a Leech labeling. Motivated by a problem in electrical engineering, the concept was introduced by John Leech in 1975 and he found five examples which are the only known Leech trees till date. We have introduce a new parameter called Leech index which gives a measure of how close a tree is towards being a Leech tree. Let f : E → {1, 2, 3, . . . } be an edge labeling of T such that both f and the corresponding path weight function w are injective. Let S denote the set of all weights of the paths in T. Let kf be the positive integer such that {1, 2, 3, . . . , kf } ⊆ S and kf + 1 ∈/ S. Then k(T) = max kf , where the maximum is taken over all such edge labelings f is called the Leech index of T. We determine the Leech index of several families of trees and obtain bounds for this parameter. The concept of Leech trees has been extended to general graphs also and studies are going in that direction too. 
Spring 2020
16 September  Marthe Bonamy, LaBRI  On Vizing's edge colouring question 
In his 1965 seminal paper on edge colouring, Vizing proved that a (Delta+1)edge colouring can be reached from any given proper edge colouring through a series of Kempe changes, where Delta is the maximum degree of the graph. He concludes the paper with the following question: can an optimal edge colouring be reached from any given proper edge colouring through a series of Kempe changes? In other words, if the graph is Deltaedgecolourable, can we always reach a Deltaedgecolouring? If true, this would imply a more recent conjecture of Mohar (2006) that in any graph, all (Delta+2)edgecolourings are equivalent up to a series of Kempe changes. We discuss recent progress around these questions.  
23 September  Carla Groenland, University of Oxford  Asymptotic dimension of graph classes 
I will attempt to give some intuition for asymptotic dimension of graph classes, starting from the definition and giving the connection to clustered colouring and weakdiameter network decompositions. I will sketch the ideas behind some of our results, e.g. that the class of planar graphs has asymptotic dimension 2. Asymptotic dimension was introduced by Gromov in 1993 within geometric group theory and in the graph setting makes sense for infinite graphs or graph classes. Joint work with M. Bonamy, N. Bousquet, L. Esperet, F. Pirot, and A. Scott.  
30 September  Karen Yeats, University of Waterloo  Chains of hourglasses and the graph theory of their Feynman integrals 
I will give a set up for understanding the graph theory of Feynman integrals that I have been using over the last number of years. It is all very combinatorial and does not require any understanding of physics or analysis. Then I will explain about some joint work with Oliver Schnetz where we are able to complete a generalized denominator reduction on a particular family of graphs and from this calculate an arithmetic graph invariant on them known as the c_2 invariant as well as understand properties of their Feynman integrals.  
7 October  Shanise Walker, University of Wisconsin Eau Claire  The Game of Cycles 
The Game of Cycles was introduced by Francis Su in his book titled Mathematics for Human Flourishing (2020). The game is played on a simple connected planar graph together with its bounded cells, and players take turns marking edges with arrows according to a sinksource rule. The object of the game is to produce a cycle cell–a cell surrounded by arrows all cycling in one direction–or to make the last possible move. In this talk, we explore the twoplayer game for various classes of graphs and determine who has a winning strategy.  
14 October  Laura Escobar, Washington University in St. Louis  Which Schubert varieties are Hessenberg varieties? 
Schubert varieties are subvarieties of the flag variety whose geometry can be understood using the combinatorics of permutations. Hessenberg varieties are also subvarieties of the flag variety with connections to both algebraic combinatorics and representation theory. After introducing and motivating these varieties, I will discuss joint work with Martha Precup and John Shareshian in which we investigate which Schubert varieties in the full flag variety are Hessenberg varieties.  
21 October  Inês Rodrigues, University of Lisbon  An action of the cactus group on the shifted tableau crystal 
The shifted tableau crystal is a crystallike structure on shifted semistandard tableaux, introduced by Gillespie, Levinson and Purboo (2017). We define on this structure a shifted version of the crystal reflection operators, originally introduced by Lascoux and Schützenberger on type A crystals, which coincide with the restrictions of the shifted Schützenberger involution on primed intervals of two adjacent letters. Unlike type A crystal they not yield an action of the symmetric group. Following a similar approach as Halacheva (2016), we then exhibit a natural action of the cactus group on the shifted tableau crystal, realized by the restrictions of the shifted Schützenberger involution to all primed intervals of the underlying crystal alphabet, including, in particular, the crystal reflection operators.  
28 October  Jana Novotná, University of Warsaw/Charles University  Cliquewidth of Atoms and Coloring for Hereditary Graphs 
Cliquewidth is a wellestablished graph parameter. Many NPcomplete graph problems are polynomially solvable on graph classes of bounded cliquewidth. Moreover, several of these problems are polynomially solvable on a hereditary graph class G if they are so on the atoms (graphs with no clique cutset) of G. For these reasons, the boundedness of cliquewidth has been investigated for many graph classes. In this talk, we consider graph classes defined by one or two forbidden induced subgraphs. We discuss the coloring problem and the boundedness of cliquewidth of their atoms.  
4 November  Karen Meagher, University of Regina  Erdős–Ko–Rado Theorems for Groups 
The first half of this talk will be a gentle introduction to the Erdős–Ko–Rado (EKR) Theorem. This is a theorem that determines the size and structure of the largest collection of intersecting sets. It has become a cornerstone of extremal set theory and has been extended to many other objects. I will show how this result can be proven using techniques from algebraic graph theory. In the second half of this talk I will give more details about extensions of the EKR theorem to permutations. Two permutations are intersecting if they both map some i to the same point (so σ and π are intersecting if and only if σ^{−1}π has a fixed point). In 1977, Deza and Frankl proved that the size of a set of intersecting permutations is at most (n1)!. It wasn't until 2003 that the structure of sets of intersecting permutations that meet this bound was determined. For any group action, we can ask what is the size and structure of the largest set of intersecting permutations. I will show some recent results for different groups, with a focus on 2transitive groups. 

12 November 2:303:20pm  Patricia Hersh, University of Oregon  Posets arising as 1skeleta of simple polytopes, the nonrevisiting path conjecture, and poset topology 
Held jointly with the SFU Operations Research Seminar Note the nonstandard time and day of the week  
Abstract: Given a polytope P and a nonzero vector c, the question of which point in P has largest inner product (or smallest inner product) with c is the main goal of linear programming in operations research. Key to efficiency questions regarding linear programming is the directed graph G(P,c) on the 1skeleton of P obtained by directing each edge e(u,v) from u to v for c(u) < c(v) and in particular the diameter of G(P,c). We will explore the question of finding sufficient conditions on P and c to guarantee that no directed path ever revisits any polytope face that it has left; this is enough to ensure that linear programming is efficient under all possible choices of pivot rule. It turns out that posettheoretic techniques and poset topology can help shed light on this question. In discussing work on this question, we will provide background along the way.  
18 November  Natasha Morrison, University of Victoria  Hypergraph Lagrangians: Extremal Combinatorics and the FranklFüredi conjecture 
In this talk we explore the origins of hypergraph Lagrangians and their applications to classical problems in extremal combinatorics. We then focus on the FranklFüredi conjecture, which concerns the maximal value of the Lagrangian of an runiform hypergraph with m edges. In particular, we show that the conjecture is false in general, but holds for all sufficiently large 3uniform hypergraphs. This talk is based on joint work with Vytautas Gruslys and Shoham Letzter.  
25 November  Amélie Trotignon, Johannes Kepler University  Discrete harmonic functions in the threequarter plane 
Harmonic functions play an important role in probability theory and are strongly related to the enumeration of walks. Doob htransform is a way to build conditioned random processes in cones from a random process and a positive harmonic function vanishing on the boundary of the cone. Finding positive harmonic functions for random processes is therefore a natural objective in the study of confined random walks. There are very few ways to compute discrete harmonic functions. In this talk we are interested in positive discrete harmonic functions with Dirichlet conditions in three quadrants. Whereas planar lattice (random) walks in the quadrant have been well studied, the case of walks avoiding a quadrant has been developed lately. We extend the method in the quarter plane – resolution of a functional equation via boundary value problem using a conformal mapping – to the threequarter plane applying the strategy of splitting the domain into two symmetric convex cones. We obtain a simple explicit expression for the algebraic generating function of harmonic functions associated to random walks avoiding a quadrant.  
2 December  Sofiat Olaosebikan, University of Glashow  Algorithmics of the StudentProject Allocation problem 
Matching problems are all around us – they arise when we try to find the “best” allocation between two sets of entities. For example, matching transplant patients with organ donors, allocating junior doctors to hospitals, students to projects, and wireless networks to users. This class of problems was first studied in 1962 by Gale and Shapley, and its founding researchers were awarded a Nobel prize in 2012. My talk is centred around the StudentProject Allocation problem (SPA), which, in its natural form, involves finding a manytoone matching of students to projects offered by lecturers, based on student preferences over projects and the maximum number of students that each project and lecturer can accommodate. During my PhD, I explored SPA with lecturer preferences over Projects (SPAP), SPA with lecturer preferences over Students (SPAS), and SPAS with Ties (SPAST). The solution concept that we seek in this context is a “stable matching” – a matching that ensures that no student and lecturer who are not paired together would rather be assigned to each other than remain with their current assignees. In my talk, I will present the structural and algorithmic results arising from my research on finding stable matchings in the problem models mentioned above.  
8 December  Aparna Lakshmanan, St. Xavier's College  On Leech Labeling 
Note the nonstandard day of the week  
A tree is called a Leech tree, if one can assign positive integer edge weights such that the nC2 path weights are exactly 1, 2,..., nC2, where the weight of a path p, w(P) is the sum of weights of all its edges and such a labeling is called a Leech labeling. Motivated by a problem in electrical engineering, the concept was introduced by John Leech in 1975 and he found five examples which are the only known Leech trees till date. We have introduce a new parameter called Leech index which gives a measure of how close a tree is towards being a Leech tree. Let f : E → {1, 2, 3, . . . } be an edge labeling of T such that both f and the corresponding path weight function w are injective. Let S denote the set of all weights of the paths in T. Let kf be the positive integer such that {1, 2, 3, . . . , kf } ⊆ S and kf + 1 ∈/ S. Then k(T) = max kf , where the maximum is taken over all such edge labelings f is called the Leech index of T. We determine the Leech index of several families of trees and obtain bounds for this parameter. The concept of Leech trees has been extended to general graphs also and studies are going in that direction too. 
All the remaining sessions cancelled due to the coronavirus disease (COVID19)  
10 March  David Wood, Monash University  Graph and Poset Dimension Parameters 
3 March  Alexander Pott, Otto von Guericke University Magdeburg  Vanishing flats: a combinatorial viewpoint on the nonlinearity of functions 
25 Feburary  Mehdi Salimi, SFU  PursuitEvasion Games 
18 Feburary  Jason Schoeters, University of Bordeaux  Temporal cliques admit sparse spanners 
11 Feburary  Timothy Chan, Monash and Warwick  Matroid branchdepth and integer programming 
4 Feburary  (Cancelled due to campus snowfall closure)  
28 January  Joris van der Hoeven, CNRS (Distinguished PIMS Discrete Math talk) 
Integer multiplication in time O(n log n) 
21 January  Joris van der Hoeven, CNRS  Historic algorithms for integer multiplication 
14 January 
Kevin Halasz, SFU  Near transversals of latin squares 
2019 Talks
Fall 2019
03 December  Stephanie van Willigenburg, UBC 
The epositivity of chromatic symmetric functions 
26 November  Peter Horak, University of Washington 
Extremal problems on cycles in graphs 
19 November  Thaís Bardini, SFU 
Fault tolerance in cryptographic applications using coverfree families 
12 November  Brin Chan, UBC 
A generalization of balanced tableaux and matching problems with unique solutions 
05 November  Pavol Hell, SFU  Graphs with possible loops 
29 October  Kathryn Nurse, SFU 
Maximum Directed Linear Arrangement 
22 October  Robert Samal, Charles University 
The binary paint shop problem (aka necklace splitting problem) 
15 October  Farid Aliniaeifard, UBC 
Combinatorial Hopf algebras and representation theory 
08 October  Rikke Marie Langhede, DTU 
The weak group connectivity number versus the group connectivity number 
01 October  Petr Lisonek, SFU 
Maximally nonassociative quasigroups 
24 September  Gary MacGillivray, University of Victoria 
Switching (m,n)edgecoloured graphs 
17 September  Gonzalez Hermosillo de la Maza, SFU 
Active cops and robbers 
10 September 
Peter Bradshaw, SFU 
Graphs with high cop number 
03 September  Curtis Bright, University of Waterloo 
SAT Solving with Computer Algebra: A Powerful Combinatorial Search Method 
Spring 2019
23 April  Amarpreet Rattan, SFU  Parking functions and factorizations of the full cycle. 
16 April  Igor Shinkar, SFU 
On (almost) Lipschitz mappings on the discrete hypercube 
9 April  Peter Bradshaw, SFU  Cops and Robbers on Cayley Graphs 
2 April 
Nike Dattani, HarvardSmithsonian Center for Astrophysics  Solving mathematical and machine learning problems on quantum computers 
26 March 
Peter Horak, University of Washington 
Old and New Conjectures in Tilings 
19 March  Bojan Mohar, SFU  Toward a Theory of CrossingCritical Graphs, Part I 
2018 Talks
Fall 2018
4 December 
Michael Monagan, SFU  How to factor a multivariate polynomial 
20 November  Mahdieh Malekian, SFU  Excluding small graphs as immersion 
13 November  Bojan Mohar, SFU  50 Years of the RingelYoungs Map Colour Theorem 
23 October  Jonathan Jedwab, SFU  New Constructions of Difference Matrices 
16 October 
Leionid Chindelevitch, SFU  Metabolic networks: discrete geometry and optimization 
9 October  Matt DeVos, SFU  On the colourful world of Ron Aharoni 
Summer 2018
7 August  Fuji Zhang  Spectral dynamics of graph eigenvalues 
31 July  Sebastian Gonzalez, SFU  
10 July  Yifan Jing, SFU  On the genus of complete 3uniform hypergraphs 
26 June  Andrew Berget, Western Washington U  Internal Zonotopal Algebras of Reflection Arrangements 
5 June  Cedric Chauve, SFU  MaxTiC, ranking species trees using network constraints 
29 May  Amy Wiebe, University of Washington  The Slack Realization Space of a Polytope 
22 May  James Davis, University of Richmond  Apple vs Samsung: a Mathematical Battle 
15 May 
Haiyan Chen  On the sandpile group of a polygon flower 
Spring 2018
12 April  Jephian Lin  The zero forcing process and the strong Arnold property 
31 March, Seattle  Sylvie Corteel, Kevin Purbhoo, Steph van Willigenburg, Matjaž Konvalinka 
Pacific NW Combinatorics day, UWash, Seattle 
2425 Mar, Victoria  2018 Coast Combinatorics Conference University of Victoria, Victoria, BC  
20 Mar  Ryan Hayward, Alberta  Recent results on Hex 
13 Mar  Seyyed Aliasghar Hosseini, Simon Fraser University  Cops and Robbers on Graphs of Bounded Diameter 
6 Mar  Stephen Melczer, UPenn  Eventual Positivity of Rational Function Coefficients 
27 Feb  Amanda Montejano Cantoral, Universidad Nacional Autónoma de México  Forced chromatic patterns in 2colorings of the complete graph 
20 Feb  (cancellation)  
13 Feb  No seminar (Reading week)  
6 Feb  Philippe Nadeau, Lyon/CNRS  Combinatorics of the category of noncrossing partitions 
30 Jan  Samantha Dahlberg, UBC  The epositivity of clawcontractiblefree graphs 
23 Jan  Karen Meagher, Regina  Approaches to the ErdősKoRado Theorem 
16 Jan  Jessica McDonald, Auburn  Edgecolouring planar graphs with precoloured edges 
2017 Talks
Fall 2017
Dec 5  Karen Yeats C&O Waterloo 
A little progress on the completion invariance of the c_{2} invariant 
Nov 28  Koen van Greevenbroek Math, SFU 
Contracted Difference Matrices 
Nov 21  Tony Huynh Université Libre de Bruxelles 
A tight ErdősPósa function for wheel minors 
7 Nov  Leonid Chindelevitch SFU 
Finding the rank median of three genomes 
24 Oct  Ross McConnell Colorado State 
Planar and Outerplanar Graphs 
17 Oct  Ron Aharoni Israel Institute of Technology 
Cooperative colorings 
10 Oct  Sheila Sundaram Pierrepont School 
Variations on the S_nmodule Lie_n: results and conjecture 
3 Oct  Amanda Montejano Universidad Nacional Autónoma de México 
Zerosum over Z 
26 Sept  Jehanne Dousse Universität Zürich 
Refinement of partition identities 
19 Sept  Moshe Rosenfeld U of Washington 
Starters what you know or don't care to know: old and new open problems 
12 Sept  Amelie Trotignon SFU 
Simple Walks in the Three Quarter Plane 
Summer 2017
29 Aug  David Wood Monash University 
Monotone expanders and applications 
08 Aug  Aniket Mane SFU 
A tractable variant of the Single Cut or Join distance with duplicated genes 
08 Aug 
Abhinav Shantanam SFU 
Hadwiger Number of Complements of Kneser Graphs with \alpha = 2 
01 Aug  Dirk Nowotka Kiel University 
kabelian pattern matching 
25 July  Cesar Hernandez Cruz Universidad National Automate de Mexico 
kquasitransitive digraphs of large diameter 
25 July  Samuel Simon SFU 
Linking Systems of Difference Sets 
18 July  Shuxing Li SFU 
Construction and nonexistence of strong external difference family 
11 July  Julian Sahasrabudhe U of Cambridge 
Exponential Patterns in Arithmetic Ramsey Theory 
04 July  Avi Kulkarni SFU 
Reading graphs into SAGE from an IPE diagram 
04 July  Sebastian Gonzalez Hermosillo de la Maza SFU 
Proper Orientations of Planar Bipartite Graphs 
06 Jun 
Arnaud Casteigts, LaBRI, U de Bordeaux 
Robustness in Highly Dynamic Networks 
30 May  Joao Meidanis, U of Campinas, Brazil 
Genome Matrices and the Median Problem 
23 May 
Stephen Melczer, U of Waterloo 
Effective Analytic Combinatorics in Several Variables 
16 May  Mark Kayll, U of Montana 
Derangements through the years: 170819962014 
Spring 2017
18 April  Julian Sahasrabudhe, U of Cambridge 
Exponential Patterns in Arithmetic Ramsey Theory 
28 Mar  Jonathan Jedwab, SFU 
Costas cubes 
21 Mar 
Pavol Hell, SFU 
Obstruction Certificates for Geometrically Defined Graph (and Digraph) Classes 
14 Mar  David Rowe, Universitat Mainz 
*This Talk Will Begin At 2pm* The Role of Configurations in Classical Algebraic Geometry 
7 Mar  Peter Wu, SFU 
On maximal size of 2weakly compatible split system 
28 Feb  Sean Griffin, U of Washington 
Schur positivity and labeled binary trees 
21 Feb  Bojan Mohar, SFU 
The Crossing Number of the Cone of the Graph 
7 Feb  Fiachra Knox, SFU 
Majority Colourings of Digraphs 
31 Jan  Luis Goddyn, SFU 
Parity Trackles on Surfaces and Perturbations of Bilinear Forms 
2016 Talks
Fall 2016
Date  Speaker, Affiliation  Talk Title 

20 Sep  Tony Huynh, U Libre de Bruxelles 
The matroid secretary problem for minorclosed classes and random matroids 
18 Oct  Marni Mishna, SFU 
Universality classes for weighted lattice paths: where probability and ACSV meet 
25 Oct  Jørgen BangJensen, U of Southern Denmark 
Connectivity properties of tournaments and semicomplete digraphs 
1 Nov  Seyyed Aliasghar Hosseini, SFU 
Cops and Robbers on Oriented Grids I 
8 Nov  Sebastian Gonzalez, SFU  Cops and Robbers on Oriented Grids II 
15 Nov  Samantha Dahlberg, UBC  Pattern Avoidance in Restricted Growth Functions 
22 Nov  Steffen Borgwardt, U of Colorado  The Hirsch Conjecture is true for NetworkFlow Polytopes 
29 Nov  Justin Chan, SFU  Suitable Cores And Arrays 
*Please Note Date/Location: Talk on December 5th will be held in the IRMACS Theatre on MONDAY, Dec 5th 

5 Dec  Fan Chung Graham, U of California, San Diego 
Sequences: random, structured or something in between? 
Summer 2016
10 May  Natalia GarcíaColín U Nacional Autónoma de México 
On the tolerated Tverberg number 
17 May  Jonathan Jedwab Simon Fraser University 
How many mutually unbiased bases can exist in complex space of dimension d? 
24 May  Dmitry Doryn IBS Center for Geometry and Physics 
c_{2} invariants of Feynman graphs 
31 May  —  CANCELLED 
7 Jun  —  CANCELLED 
14 Jun  César Hernández Cruz U Autónoma de Zacatecas 
A dichotomy for the kernel by Hwalks problem in digraphs 
21 Jun  Flavia Bonomo U Buenos Aires 
Threecoloring and list threecoloring graphs without induced paths on seven vertices 
28 Jun (extra) 11:30am 
Ross McConnell Colorado State University 
Algorithms on graphs arising from intersections of intervals 
28 Jun  Melissa Huggan Dalhousie University 
Conjoined games: gocut and snogo 
5 Jul (extra) 11:30am 
Mathew Francis Indian Statistical Institute, Chennai Centre 
Uniquely restricted matchings in interval graphs 
5 Jul  Hendrik Süß University of Manchester 
Frobenius splitting of toric and Tvarieties 
12 Jul  Bruce Reed McGill University 
How to determine if a random graph with a fixed degree sequence has a giant component 
19 Jul  —  CANCELLED 
26 Jul  Iain Crump Simon Fraser University 
The extended graph permanent as an affine variety 
Thursday 28 Jul 3:30pm 
Mordecai Golin Hong Kong UST 
Optimal binary comparison search trees 
Spring 2016
12 Jan  Rodolphe Garbit Université d'Angers 
On the exit time from a cone for random walks with drift 
19 Jan  Suil O Simon Fraser 
Interlacing families and their application to digraphs 
26 Jan IRMACS Theatre 1:00pm (12:00 reception) 
Kilian Raschel CNRS / U François Rabelais  Tours 
Counting quadrant walks via Tutte's invariants and transformation theory of elliptic functions 
2 Feb  Stefan Hannie Simon Fraser 
Immersion of 2regular digraphs 
9 Feb  Reading break  CANCELLED 
16 Feb  Pierre Tarrago Saarlandes University 
Categories of twocolored noncrossing partitions 
23 Feb  —  CANCELLED 
1 Mar  Cedric Chauve Simon Fraser 
Counting, generating and sampling tree alignments 
8 Mar  Julien Courtiel UBC 
Terminal chords in connected diagrams chords 
15 Mar  Claudia Linhares Sales UF Ceará 
Bcontinuity of lexicographic product of graphs 
22 Mar  Alejandro Morales UCLA 
Hook Formulas for Skew Shapes 
22 Mar (2nd talk) 3:00pm 
Barnaby Martin Middlesex University London 
The packing chromatic number of the infinite square lattice 
29 Mar  Fiachra Knox Simon Fraser 
Ramanujan gap for the norm of adjacency operators 
5 Apr    PIMSSFU Colloquium: Anthony VárillyAlvarado in IRMACS Theatre 
2015 Talks
Fall 2015
22 Sept  Ilya Shmulevich Institute for Systems Biology 
Probabilistic Boolean Networks as Models of Genetic Regulatory Networks 
Thursday 24 Sept 11:00am 
Harrison Chapman University of Georgia 
Asymptotic laws for knot diagrams 
29 Sept  Gary Greaves Tohoku University 
Equiangular lines in Euclidean spaces 
6 Oct  Karen Yeats Simon Fraser 
A few c_2 invariants of circulant graphs 
13 Oct  Abbas Mehrabian UBC/SFU 
Cops and a Fast Robber on Planar and Random Graphs 
20 Oct  Matt DeVos Simon Fraser 
Flows with large support 
27 Oct  Alejandro Erickson Durham University 
Graphs at the heart of the cloud 
3 Nov  Eric Fusy CNRS / École Polytechnique 
Baxter permutations and meanders 
10 Nov  Yota Otachi Japan Advanced Institute of Science and Technology 
Induced minorfree graphs: isomorphism and cliquewidth 
17 Nov  Theodore Kolokolnikov Dalhousie University 
Maximizing algebraic connectivity for certain families of graphs 
24 Nov  Shenwei Huang Simon Fraser University 
Colouring graphs without an induced path: A survey 
1 Dec  Luis Goddyn Simon Fraser 
Extending Hawiger’s Conjecture and Bicircular Matroids 
8 Dec  Campus closure  CANCELLED 
Monday 14 Dec PIMS 8500 1:30pm 
Chính Hoàng Wilfred Laurier University 
On the structure of (pan, even hole)free graphs 
Monday 14 Dec PIMS 8500 3:00pm 
Mark Giesbrecht U Waterloo 
Eigenvalues, invariant factors and random integer matrices (with a little bit of computing) 
Summer 2015
12 May 
Nicholas Beaton U. Saskatchewan 
Solvable selfavoiding walk and polygon models with large growth rates 
19 May  Matt DeVos Simon Fraser 
On nowherezero flows 
26 May  Luis Goddyn Simon Fraser 
Odd thrackles on surfaces 
02 June  CanaDAM 2015  CANCELLED 
09 June  Robert Samal Charles U. 
Unique Vector Coloring and Cores 
16 June  Ron Graham conference  CANCELLED 
23 June  Amanda Montejano UNAM 
Cyclic orders and oriented 3hypergraphs. 
30 June  Marni Mishna Simon Fraser 
Efficient uniform generation of reluctant walks 
07 July  Moshe Rosenfeld  The wondering salmon problem and (unrelated) graph designs 
14 July  Christopher Duffy UVic 
Colourings and simple colourings of (j,k)mixed graphs 
Thursday 23 July 
Bruce Reed McGill University 
Connectivity preserving iterative compaction 
28 July  César HernándezCruz UNAM 
Mixed cages Acyclic disconnection and minimum feedback arc sets of graphs 
04 August  Jan Manuch UBC 
Spring 2015
Tuesday Jan 13 
David Gajser, University of Ljubljana, Slovenia  Simple PTAS’s for families of graphs excluding a minor 
Tuesday Jan 20 
Martina Mockovciakova (University of West Bohemia, Pilsen, Czech Republic)  Star Edge Coloring of Some Classes of Graphs 
Tuesday Jan 27 (IRMACS, 12:30 reception) 
Michael Singer (NCSU) (PIMS Distinguished Visitor) 
Galois theory of difference and differential equations. 
Tuesday Feb 3 
Manuel Kauers (RISC) 
Walks in the Quarter Plane with Multiple Steps 
Tuesday Feb 10 
(Reading Week) 

Tuesday Feb 17 
Cedric Chauve, SFU  Graphtheoretical algorithms to correct gene trees 
Tuesday Feb 24 
Edita Rollova (KMA)  Homomorphisms and colourings of signed graphs 
Tuesday Mar 3 
Murilo da Silva  Evenholefree graphs 
Tuesday Mar 10 
Henry Liu, New University Lisbon, Portugal  Connected subgraphs in edgecoloured graphs 
Tuesday Mar 17 
Ashok Rajaraman  Vertex Ordering Problems for Hypergraphs: Connections to the Consecutive Ones Property 
Tuesday Mar 24 
Veronika Irvine (U Victoria)  Lace patterns 
Tuesday Mar 31 
Guillaume Chapuy (Liafa/ Paris 7)  Spanning trees of the graph of spanning trees. 
Tuesday April 7  Karen Yeats (SFU)  Rooted tree classes and the renormalization group equation 
Tuesday April 14  Nathan Ilten (SFU)  Counting lines on toric surfaces 
2010 Talks
Seminars 20012010
Fall 2010  
Tue Sept 14 / 2:30 / K9509  Zdenek Dvorak  Choosability of planar graphs + Welcome party 
Tue Sept 21 / 2:30 / K9509  Jessica McDonald, SFU  Characterizing high chromatic index 
Tue Sept 28 / 2:30 / K9509  Emmanuel Godard, CNRS/ U. Aix (Marseilles)  Using isometric embeddings in distributed algorithms for the median problem in partial rectangular grids and their relatives 
Tue Oct 5 / 2:30 / K9509  Mohamed Omar, UC Davis.  Graph Automorphism and Permutation Groups via Polytopes 
Tue October 12 / 2:30 / K9509  Karen Yeats, SFU  The support of power series given by systems of equations 
Tue October 19 / 2:30 / K9509  Cedric Chauve, SFU  On matrices that do not have the ConsecutiveOnes Property. 
Tue October 26 / 2:30 / K9509  Andrew Crites, U. Washington  Affine permutations and pattern avoidance 
Tue Nov 2 / 2:30 / K9509  Petr Lisonek, SFU  Nonlinear functions 
Tue Nov 9 / 2:30 / K9509  Flavio Guinez, UBC  The reconstruction of a colored grid by its projections: a solution to the 2atom problem in discrete tomography 
Tue November 16 / 2:30 / K9509  Amites Sarkar, UWW  Partitioning geometric covers 
Tue November 23 / 2:30 / K9509  Kurt Luoto, UBC  Quasisymmetric and noncommutative Schur functionsï»¿ 
Tue Nov 30 / 2:30 / K9509  Aki Avis, SFU  3Phase Golay Triads 
Tue Dec 7 / 2:30 / K9509  Pradeep Sarvepalli (UBC)ï»¿  Topological color codes over prime power alphabet 
Satruday December 11 (Western Washington University)  Combinatorial potlatch  Richard Guy, University of Calgary Christine Kelley, University of Nebraska, Lincoln KaiUwe Schmidt, Simon Fraser University 
Tue Dec 14 / 2:30 / K9509  Bruce Shepherd  Minimum Cost Networks Supporting All Matchings 
Summer 2010  
Tue 08 Jun / 2:30 / K9509  John Stardom, Statistics Canada  Sample Stratification and Allocation: an Application to Statistics Canada's Unified Enterprise Survey 
Tue 22 Jun / 2:30 / K9509  Christian Stump, CRM/ LaCIM UQAM  Noncrossing and nonnesting partitions and the cyclic sieving phenomenon 
Tue 07 July / 2:30 / K9509  Karel Casteels, SFU  Combinatorial aspects of the prime spectrum of quantum matrices 
Tue 13 July / 2:30 / K9509  Guillaume Chapuy, SFU  Covered Maps on Orientable Surfaces 
Tue 10 August / 2:30 / K9509  KaiUwe Schmidt, SFU  Appended msequences with merit factor greater than 3.34 
Spring 2010 

Tue 09 Feb / 3:30 / TASC2 8500  Kevin Milans, U. Illinois UC  Degree Ramsey Numbers of Graphs 
Tue 02 Mar / 3:30 / TASC2 8500  Ararat Harutyunyan, SFU  Independent dominating sets in graphs of girth five 
Thu 04 Mar / 1:00 / K 9509  Adolfo Rodriguez, LaCIM UQAM  Bugs, colonies, and qBoson normal ordering 
Tue 23 Mar / 3:30 / TASC2 8500  Roland Wittler, Bielefeld and SFU.  Phylogenybased Analysis of Gene Clusters 
Thu 25 Mar / 1:00 / K 9509  Justin Chan, University of Victoria  The LamkenWilson Theorem and Its Applications 
Mon 12 Apr / 2:30 / IRMACS Theatre, ASB 10900, SFU  Peter Horak, U. Washington, Tacoma  Tiling nspace by cubes 
Tue 13 Apr / 3:30 / TASC2 8500  Alejandro Erickson, University of Victoria  Negative correlation properties for graphs 
Tue 20 Apr / 3:30 / TASC2 8500  Robert Samal, Charles University, Prague  Star Chromatic Index 
Fall 2009 

Tue 22 Sep / 1:30 / K9509  Bojan Monar, SFU  How useful can the spectral radius of a graph be? 
Tue 29 Sep / 1:30 / K9509  Ladislaw Stacho  Cyclic colorings of plane graphs with independent faces 
Tue 06 Oct / 1:30 / K9509  Guillaume Chapuy  Maps on surfaces : asymptotic enumeration and metric properties. 
Tue 13 Oct / 1:30 / K9509  Andrea Spencer, SFU  Title: Efficient Edge List Colouring of Cubic Planar Graphs 
Tue 27 Oct / 1:30 / K9509  Mahdad KhatirinejadFard, Helsinki U. of Tech.  Title: Edgeweighting vertex colouring of (di)graphs 
Sat 21 Nov / 10:00 / Labatt Room  2009 Combinatorial Potlach, SFU Harbour Centre  Speakers: TBA 
Summer 2009 

Tue 05 May / 2:00 / TASC2 8500  Daniel Kral, Charles U., Prague  Removal Lemma for systems of linear equations 
Tue 25 Aug / 3:30 / K9509  Gena Hahn, U. de Montreal  Cops and Robbers on Infinite Graphs 
Tue 01 Sep / 3:30 / K9509  Robert Samal, Charles U., Prague  Cubical chromatic number, spherical chromatic number, and Lovasz theta function 
Spring 2009  
Tue 10 Feb / 3:30 / K9509  Robert Bailey, Carleton University  Packing spanning trees in graphs and bases in matroids 
SatSun 2122 Feb, Univ. of Victoria  11th Coast Combinatorics Conference  
Tue 24 Feb / 3:30 / K9509  Markus Grassl, Centre for Quantum Technologies (NUS), Singapore  Searching for good errorcorrecting codes 
Tue 31 Mar / 3:30 / K9509  Hernando Bermudez, University of Montana  Sheduling Tournaments with Home and Away Constraints 
Tue 24 Mar / 3:30 / K9509  Natasa Przulj, UC Irvine  From Network Topology to Biological Function and Disease 
Tue 14 Apr / 3:30 / K9509  Cedric Chauve  Enumerative results for graph decompostion trees 
Tue 21 Apr / 3:30 / K9509  Eric Fusy  Schnyder woods generalized to higher genus surfaces 
Tue 28 Apr / 3:30 / K9509  Bruce Richter, University of Waterloo  Do we really know what characterizes planarity? 
Fall 2008 

Tue 23 Sep / 3:30 / K9509  Juanjo Ru\'e, Politechnical U. of Catalonia  On a question of S\'arkozy and S\'os on bilinear forms 
Thu 02 Oct / 1:30 / K9509  Mireille BousquetMelou, CNRS, Bordeaux  Enumeration of colored planar maps 
Tue 14 Oct / 3:30 / K9509  Eli Berger, Univeristy of Haifa  Using classical topology in combinatorial problems. 
Tue 28 Oct / 2:30 / ASB 10900  David Brydges, CRC Chair, UBC  Combinatorics with Gaussian Integreals 
Tue 04 Nov / 3:30 / K9509  Kenichi Kawarabayashi, Nat. Inst. of Informatics, Japan  From the Plane to Higher Surfaces 
Tue 18 Nov / 3:30 / K9509  Aida Ouangraoua, SFU  Algorithmic tools to compare treestructured biological objects 
Sat 22 Nov / 10:00 / U.P.S.  2008 Combinatorial Potlach, U. Puget Sound, Tacoma  Speakers: E. Fusy + C. Dunn + I. Dumitriu 
Tue 02 Dec / 3:30 / K9509  Brian Alspach, U. Newcastle  Hamilton paths in vertextransative graphs 
Summer 2008 

Tue 06 May / 3:30 / K9509  Bojan Mohar, SFU  On the sum of k largest eigenvalues of graphs and symmetric matrices 
Tue 20 May / 3:30 / K9509  Simon Spacapan, SFU  The Acyclic, the Star, and the Degenerate Chromatic Numbers 
Tue 22 Jul / 3:30 / K9509  Winfried Hochstaettler, U. of Hagen, Germany  Flows in Oriented Matroids and a Chromatic Number 
Tue 19 Aug / 2:30 / TASK 9204  David Coudert, CNRS U. Nice Sophia  Survivability and Routing in Oriented Networks 
Spring 2008 

Tue 15 Jan / 3:30 / K9509  Maia Fraser, U. Colima, Mx  Local routing on higher surfaces 
Tue 25 Feb / 3:30 / K9509  Gabor Kun, Etvos University, Hungary  Cops and robbers in random graphs 
Tue 04 Mar / 3:30 / K9509  Luis Goddyn, Simon Fraer University  How bad can a Euclidean Traveling Salesman Problem be? 
Tue 11 March / 3:30 / TASC2 8500  Penny Haxell, University of Waterloo  More on Sperner's Lemma 
Tue 25 March / 3:30 / K9509  Agelos Georgakopoulos, University of Hamburg  Infinite cycles in graphs 
Tue 25 March / 3:30 / K9509  Marni Mishna, Simon Fraser University  Decomposition and Enumeration of Planar Graphs, Part I: Combinatorial Warm Up 
Tue 01 April / 3:30 / K9509  Eric Fusy, Simon Fraser University  Decomposition and Enumeration of Planar Graphs, Part II 
Tue 08 April / 3:00 / K 9509  Laura Chavez, Simon Fraser University  Flow and chromatic numbers for matroids (reception to follow) 
Tue 15 April / 3:30 / K 9509  Derek Bingham, Simon Fraser University  Defining contrast subspaces and the randomization structure of factorial designs 
Tue 29 April / 3:30 / K 9509  Sean McGuinness, Thompson Rivers University  Perfect Path Double Covers, Matroids, A Theorem of Fan, ... And A Problem of Goddyn. 
Fall 2007 

Tue 25 Sep / 10:30 / ASB10900  Luis Goddyn, SFU (Coast to Coast)  Spectra of (3,6)Fullerenes 
Tue 25 Sep / 2:30 / TASC2 8500  Mark Siggers, SFU  Integer packings of hypergraphs through fractional packings 
Sat 29 Sep / 11:00 / U. Victoria  2007 Combinatorial Potlach  Speakers: Perkel + Moon + Quas 
Tue 02 Oct / 2:30 / TASC2 8500  Robert Samal, SFU  Trading Handles for Crossings 
Tue 09 Oct / 2:30 / TASC2 8500  Blair D Sullivan, Princeton  Feedback Arc Sets in Circular Interval Digraphs 
Tue 23 Oct / 2:30 / TASC2 8500  Gabor Kun, SFU  An asymptotic version of the BollobasCatlinEldridge conjecture 
Tue 30 Oct / 2:30 / TASC2 8500  Matthias Koeppe, U. Magdeburg, de  Topic: Ehrhart polynomials of matroid polytopes 
Tue 06 Nov / 2:30 / TASC2 8500  Eric Fusy, SFU  Transversal structures on triangulations: combinatorial study and algorithmic applications 
Tue 13 Nov / 2:30 / TASC2 8500  Vladmir Korzhik, Nat. U. Chernivtsi, Ukraine  Some open problems on the set of triangular embeddings of a complete graph 
Tue 27 Nov / 2:30 / TASC2 8500  Frederic Giroire, Intel Research  Probabilistic algorithms to compute the cardinality of very large datasets 
Summer 2007 

Tue 15 May / 3:30 / ASB10908  Ambat Vijayakumar  Graph operators and cographs: some recent trends 
Fri 18 May / 1:30 / ASB10900  Andre Raspaud  Injective Colourings of Graphs 
Tue 22 May / 3:30 / ASB10908  Jonathan Jedwab, SFU  Golay complementary sequences: a multidimensional approach 
Thu 24 May / 11:30 / ASB10900  Janos Pach  Decomposition of multiple coverings 
Fri 25 May / 2:30 / ASB10900  Herbert Wilf  Mathematics: An experimental science 
Fri 31 Aug / 2:30 / 1430 SFU Harbour Centre  Paul Seymour  Structure Theorems in Graph Theory 
Spring 2007 

Tue 30 Jan / 3:30 / ASB10908  Azhvan SheikhAhmady, SFU  Eigenvalues of Cayley graphs and group characters. 
Tue 6 Feb / 3:30 / ASB10908  Cedric Chauvre, SFU  Inferring ancestral genomes with common intervals 
Tue 20 Feb / 3:30 / ASB10908  Neil Robertson, Ohio State U.  CANCELLED 
SatSun 2425 Feb /9:30 / U. Victoria  Eighth Coast Combinatorics Conference  Speakers: Several 
Tue 27 Feb / 3:30 / ASB10908  Gena Hahn, Universite de Montreal  CANCELLED 
Tue 06 Mar / 3:30 / ASB10908  Francois Bergeron, UQAM  A combinatorial classification of polynomials 
Tue 13 Mar / 3:30 / ASB10908  Mike Zabrocki, York U.  A Zoo of Hopf Algebras 
Tue 20 Mar / 3:30 / ASB10908  Robert Samal, SFU  On XY mappings 
Tue 27 Mar / 3:30 / ASB10908  Jonathan Jedwab, SFU  On XY mappings 
Fall 2006 

Tue 26 Sep / 3:30 / ASB10908  Tamon Stephen, SFU  Colourful Simplicial Depth 
Wed 27 Sep / 3:30 / ASB9705  Eyal Ackerman, SFU  Maximum size of a $k$quasiplanar graphs 
Tue 10 Oct / 3:30 / (UBC) WMAX110  Joachim Rosenthal, U. Zurich  Three challenges of Claude Shannon 
Tue 17 Oct / 3:30 / ASB10908  Tomas Kaiser, U. West Bohemia  Nonintersecting perfect matchings in cubic graphs 
Tue 31 Oct / 2:30 / ASB10900  Dragos Cvetkovic, U. of Belgrade  Signless Laplacians of finite graphs 
Sat 11 Nov / 10:00 / Portland State U.  2006 Combinatorial Potlach  Speakers: Brualdi + Gordon + DeVos 
Tue 21 Nov / 3:30 / ASB10900  Kevin Purbhoo, UBC  Puzzles, Tableaux, and Mosaics 
Tue 05 Dec / 3:30 / (UBC) WMAX110  John Gimbel, University of Alaska  Remarks on split graphs and related notions 
Tue 12 Dec / 2:30 / ASB10900  Moshe Rosenfeld, University of Washington, Tacoma  Spanning Graph Designs, an extension of the "Graph disease" 
Tue 12 Dec / 3:30 / ASB10900  John Irving, St. Mary's University  Counting Transitive Factorizations in the Symmetric Group 
Summer 2006 

Tue 7 May / 3:30 / ASB10908  Winfried Hochstaettler, Fern Univ at Hagen  Online matching on a line 
Thu 22 Jun / 9:30 / ASB10900  SFU Discrete Math Day  Seven talks by distinguished visitors 
Thu 29 Jun / 2:30 / AQ 3149  Winfried Hochstaettler, Fern Univ at Hagen  On the game chromatic index of trees 
Tue 4 Jul / 3:30 / K9509  Brian Lucena, American Univ. in Cairo  Minimal forbidden minors for treewidth 
Tue 8 Aug / 3:30 / ASB10900  Jacob Fox  Ramseytype results for intersection graphs:w 
Spring 2006  
Tue 24 Jan / 3:30 / ASB10900  Maria Chudnovski, Princeton U.  The structure of bullfree graphs 
Tue 14 Feb / 3:30 / ASB10900  Luis Goddyn, SFU  Silver Cubes 
Tue 28 Feb / 3:30 / (UBC) WMAX216  Shakhar Smorodinsky, Courant Inst.  On Ksets in dimensions 2, 3 and 4. 
Tue 14 Mar / 3:30 / ASB10900  Arie Bialostocki, University of Idaho  Some Problems in View of Recent Developments of the Erdos Ginzburg Ziv theorem 
Tue 21 Mar / 3:30 / ASB10900  Susan Barwick, University of Adelaide  The geometry of linear perfect hash families 
Tue 28 Mar / 3:30 / (UBC) WMAX216  Kee Yuen Lam, UBC  The combinatorics of sums of squares as studied via topology 
Tue 4 Apr / 3:30 / ASB10908  Aidan Roy, University of Waterloo  Equiangular lines, mutually unbiased bases, and difference sets 
Fall 2005  
Tue 13 Sep / 3:30 / (UBC) WMAX216  Gabor Tardos, SFU  Toward and Extremal Theory of Ordered Graphs 
Tue 20 Sep / 3:30 / ASB10908  Brian Alspach, University of Regina  Time constrained searching and sweeping 
Tue 27 Sep / 3:30 / ASB10908  Richard Anstee, UBC  Forbidden Configurations: An update 
Tue 04 Oct / 3:30 / EAA1100  Matt DeVos, SFU  Pentagon Colouring 
Tue 11 Oct / 3:30 / ASB 10900  Bojan Mohar, SFU  Quadrangulations, Eulerian triangulations, and 5critical graphs 
Tue 25 Oct / 3:30 / (UBC) WMAX216  David Kirkpatrick, UBC  Minimizing precision/input in the evaluation of geometric primitives 
Tue 08 Nov / 3:30 / (UBC) WMAX216  Luis Goddyn, SFU  An Optimization Problem Arising from VideoonDemand Broadcasting 
Wed 09 Nov / 3:30 / ASB10908  Santosh Kabadi, U New Brunswick  Integer Exact Network Synthesis Problem 
Tue 15 Nov / 3:30 / ASB10908  Jonathan Jedwab, SFU  Proof of the Barker Array Conjecture 
Sat 19 Nov / 10:30 / Seattle U.  2005 Combinatorial Potlach  Speakers: Mohar + Quinn + Caughman 
Summer 2005  
Tue 03 May / 3:30 / EAA1100  Cedric Chauve, Universite du Quebec a Montreal  Conservation of combinatorial structure in genome rearrangement scenarios 
Tue 17 May / 3:30 / EAA1100  Rados Radoicic, Rutgers University  Intersection patterns of geometric objects 
Tue 31 May / 3:30 / EAA1100  Danny Dyer, University of Regina  Digraphs with small sweep number 
Tue 21 Jun / 3:30 / EAA1100  Anna Galluccio, IASICNR, Roma, Italy  Regular Join Graphs 
Tue 28 Jun / 3:30 / ASB 10900  CANCELLED: Mordecai Golin, Hong Kong U. of Science  The KnuthYao QuadrangleInequality & TotalMonotonicity 
Wed 06 Jul / 3:30 / ASB 10900  Ebad Mahmoodian, Sharif University  Problems and Motivations 
Tue 19 Jul / 3:30 / EAA1100  Ron Aharoni, Technion Institute, Israel  Topological Methods in Matching Theory 
Spring 2005  
Tue 18 Jan / 3:30 / EAA1100  Colin Percival, University of Oxford  Matching with mismatches 
Thu 10 Feb / 3:30 / EAA1100  Peter Horak, University of Washington  Completing Latin squares 
Tue 15 Feb / 3:30 / EAA1100  Binay Bhattacharya, Simon Fraser University  Collection depots problem on networks 
Tue 22 Feb / 3:30 / EAA1100  Evangelos Kranakis, Carleton University  The Mobile Agent Rendezvous Problem 
Mon 28 Feb / 4:00 / K9509  Chris Mitchell, Royal Holloway  Cryptanalysis of methods for combining confidentiality and integrity 
Tue 1 Mar / 3:30 / EAA1100  Nati Linial, U. Washington & Hebrew University  Finite Metric Spaces  A quick overview 
Tue 15 Mar / 3:30 / EAA1100  Frank Fiedler, Simon Fraser University  The Search for More Golay Sequences 
Tue 22 Mar / 3:30 / EAA1100  Petr Lisonek, Simon Fraser University  Enumeration of Codes of Fixed Cardinality up to Isomorphism 
Tue 29 Mar / 4:30 / AQ 3182  Laszlo Lovasz, Microsoft Research and Eotvos Lorand University  Graph homomorphisms, statistical physics, and limits of graph sequences 
Tue 05 Apr / 3:30 / EAA1100  Boaz Benmoshe, Simon Fraser University  Guarding 1.5D terrains 
Fall 2004  
Tue 21 Sep / 2:30 / K9509  Abraham P. Punnen, University of New Brunswick  The number of distinct tour values for the traveling salesman problem: one, two, three, all 
Tue 28 Sep / 3:30 / EAA1100  Arash Rafiey, University of London  Characterization of edgecolored complete graphs with properly colored Hamilton paths 
Tue 5 Oct / 3:30 / EAA1100  Frank Fiedler, Simon Fraser  On the degree of Mathon maximal arcs 
Tue 12 Oct / 3:30 / EAA1100  Jonathan Jedwab, Simon Fraser  A survey of the merit factor problem for binary sequences 
Tue 19 Oct / 3:30 / EAA1100  Ladislav Stacho, Simon Fraser  Asymptotics of random RNA 
Tue 26 Oct / 3:30 / EAA1100  Jeffrey Farr, Simon Fraser  Large caps with free pairs in PG(N,q) 
Tue 2 Nov / 3:30 / EAA1100  Mohammad Ghebleh, Simon Fraser  Circular chromatic number of evenfaced torus graphs 
Tue 9 Nov / 3:30 / EAA1100  Petr Lisonek, Simon Fraser  Variable rate linear codes with an application to steganography 
Tue 16 Nov / 3:30 / EAA1100  Luis Goddyn, Simon Fraser  Two results for the Lonely Runner 
Sat 20 Nov / 10:30 / Harbour Centre  Combinatorial Potlatch 2004  
Tue 23 Nov / 3:30 / EAA1100  Pavol Hell, Simon Fraser  Matrix partitions of perfect graphs 
Tue 30 Nov / 3:45 / EAA1100  Xuding Zhu, National Sun Yatsen University  Circular coloring of Kneser graphs, Schrijver graphs and cones over graphs 
Tue 7 Dec / 3:30 / EAA1100  Victor Dalmau, Universitat Pompeu Fabra  Constraint Satisfaction Problems and Expressiveness 
Summer 2004  
Tue 18 May / 3:30 / EAA1100  Daniel Kral, Charles University  Circular edgecolorings of graphs with large girth 
Fri 28 May / 11:30 / EAA1100  Peter Pleasants, University of Queensland  Almost disjoint families of 3term arithmetic progressions 
Fri 28 May / 2:30 / K9509  Nantel Bergeron, York University  Combinatorial Hopf algebras 
Mon 31 May / 2:30 / K9509  Bojan Mohar, University of Ljubljana  Graph minors and graphs on surfaces 
Tue 29 Jun / 2:30 / K9509  Marni Mishna, LaBRI, University of Bordeaux  On the utility of effective Dfinite symmetric functions in combinatorics 
Tue 13 Jul / 3:30 / EAA1100  Bruce Reed, McGill University  Vertex Colouring Edge Weightings 
Tue 20 Jul / 2:30 / K9509  Nadya Shirokova, Inst. Hautes Etudes Sci.  Space approach to invariants for families 
Tue 27 Jul / 10:00 / Halpern Centre, Room 126  Roman Nedela, Slovak Academy of Sciences  Regular maps  combinatorial objects relating different fields of mathematics 
Tue 10 Aug / 3:30 / EAA1100  Moshe Rosenfeld, University of Washington  Prisms over graphs 
Tue 17 Aug / 3:30 / EAA1100  Tomas Kaiser, University of West Bohemia  Eulerian colorings of graphs 
Spring 2004  
Tue 20 Jan / 3:30 / EAA1100  Jim Davis, Math & CS, U of Richmond  Polynomial arithmetic behind the IEEE 802.12 standard for 100Mbit/s data transmission 
Tue 27 Jan / 3:30 / EAA1100  Jeffrey Farr, Mathematics, SFU  A new polynomial construction for some linear codes 
Tue 03 Feb / 3:30 / EAA1100  Petr Lisonek, Mathematics, SFU  Binary caps with many free pairs of points 
Tue 10 Feb / 3:30 / EAA1100  Jan Manuch, Computing, SFU  Inverse protein folding in 2D HP model 
Tue 17 Feb / 3:30 / EAA1100  SEMINAR CANCELLED  Reading break 
Tue 24 Feb / 3:30 / EAA1100  Ladislav Stacho, Mathematics, SFU  Traversal of quasiplanar subdivisions without using mark bits 
Tue 01 Mar / 3:30 / EAA1100  Chris Mitchell, U of London, UK  Universal cycles of permutations 
Tue 02 Mar / 3:30 / EAA1100  Mahdad Khatirinejad Fard , Mathematics, SFU  ICColorings and ICindices of graphs 
Thu 11 Mar / 11:30 / K9509 (!)  Juergen Bierbrauer, Michigan Technological University  Cyclic codes and their applications 
Tue 16 Mar / 3:30 / EAA1100  Veselin Jungic, Mathematics, SFU  Rainbow Ramsey Theory 
Tue 23 Mar / 3:30 / EAA1100  Van Vu, Mathematics, UCSD  ErdosFolkman conjecture 
Tue 30 Mar / 3:30 / EAA1100  Luis Goddyn, Mathematics, SFU  From graceful labelings to Heawood embeddings 
Tue 06 Apr / 3:30 / EAA1100  Jozsef Solymosi, Mathematics, UBC  Applications of hypergraph regularity 
Fall 2003  
Tue 30 Sep / 3:30 / EAA1100  Jan Manuch, Math & CS, SFU  On fwise Arc Forwarding Index and Wavelength Allocations in Faulty Alloptical Networks 
Tue 07 Oct / 3:30 / EAA1100  Luis Goddyn, Mathematics, SFU  Packing Groupnonvanishing Apaths 
Tue 14 Oct / 3:30 / EAA1100  Ladislav Stacho, Mathematics, SFU  Closure for the Property of Having a Hamiltonian Prism 
Tue 21 Oct / 3:30 / EAA1100  Petr Lisonek, Mathematics, SFU  A New Application of the Linear Programming Method in Coding Theory 
Tue 04 Nov / 3:30 / EAA1100  Russel Martin, Computing, U of Warwick  Rapidly Mixing Markov Chains for Sampling Contingency Tables with a Constant Number of Rows 
0810 Nov / / UVic  Combinatorial Potlatch and 5th Coast Combinatorics Conference  
Thu 13 Nov / 3:30 / EAA1100  Zdenek Ryjacek, Mathematics, U of West Bohemia  Closure Concepts, Stable Properties and Forbidden Subgraphs for Hamiltonicity 
Tue 18 Nov / 3:30 / AQ 3149  Helaman and Claire Ferguson  Mathematics in Stone and Bronze (The Alan Mekler Lecture) 
Tue 25 Nov / 3:30 / EAA1100  Alireza Hadj Khodabakhshi, Computing, SFU  A Fast Algorithm for Finding a Global Alignment Between Two Genomes 
Spring 2003  
Tue 07 Jan / 3:30 / EAA1100  Reza Naserasr, Mathematics, SFU  Homomorphisms of Planar Graphs 
Tue 14 Jan / 3:30 / EAA1100  Pavol Hell, Mathematics/Comp. Sci., SFU  Interval Bigraphs and Circular Arc Graphs 
Tue 21 Jan / 3:30 / EAA1100  Romeo Rizzi, University of Trento (Italy)  Permutation Routing on POPS Networks 
Tue 28 Jan / 3:30 / K9509 (!)  Nantel Bergeron, Mathematics and Statistics, York University  Quasisymmetric Polynomials and TemperleyLieb Invariants and Covariants 
Tue 11 Feb / 3:30 / EAA1100  Tom Friedetzky, Comp. Sci., SFU  A Proportionate Fair Scheduling Rule with Good Worstcase Performance 
Tue 18 Feb / 3:30 / EAA1100  Jan Manuch, SFU  Equations on Words and Defect Theorems 
Tue 25 Feb / 10:30 (!) / K9509  Wendy Myrvold, University of Victoria  Hunting for Torus Obstructions 
Tue 25 Feb / 3:30 / EAA1100  Jaroslav Nesetril, Charles University, Prague  Ramsey Classes and Homogeneous Graphs 
Tue 04 Mar / 3:30 / EAA1100  Veselin Jungic, Mathematics, SFU  Colorings of Plane Graphs with No Rainbow Faces 
Tue 11 Mar / 3:30 / EAA1100  Jessie Zhu, SFU  Phylogenetics Incorporating Stratophenetics 
Tue 18 Mar / 3:30 / EAA1100  Riste Skrekovski, PIMS/SFU  Coloring 1001 Maps on Surfaces 
Tue 25 Mar / 3:30 / EAA1100  Stephanie van Willigenburg, Mathematics, U. British Columbia  Acyclic Orientations and Excedence Permutations 
Tue 15 Apr / 10:30 (!) / ASB 9896 (!)  Yuri Gurevich, Microsoft Research  What is Polynomial Time Anyway? 
Tue 15 Apr / 3:30 / EAA1100  Joel Friedman, Mathematics, U. British Columbia  A Proof of Alon's Second Eigenvalue Conjecture 
Tue 29 Apr / 3:30 / EAA1100  Michael Molloy, Microsoft Research and University of Toronto  Random Constraint Satisfaction Problems, aka Homomorphisms of Random Hypergraphs 
Fall 2002  
Tue 10 Sep / 2:00 / EAA1100  Juan Jose MontellanoBallesteros, UNAM Mexico  Polychromatic Cliques 
Tue 17 Sep / 2:00 / EAA1100  Ladislav Stacho, Mathematics, SFU  Edgedisjoint Spanners in Multidimensional Torus 
Tue 24 Sep / 2:00 / EAA1100  Jan Manuch, SFU  On the Descriptional and Computational Complexities of Infinite Words 
Tue 01 Oct / 2:00 / EAA1100  Tom Brown, Mathematics, SFU  On the Canonical Version of a Simple Theorem in Ramsey Theory 
Tue 08 Oct / 2:00 / EAA1100  Petr Lisonek, Mathematics, SFU  A Family of Complete Caps in PG(n,2) 
Tue 15 Oct / 2:00 / EAA1100  Riste Skrekovski, PIMS/SFU  Cyclic, Diagonal, and Facial Colorings 
Tue 22 Oct / 2:00 / EAA1100  Veselin Jungic, Mathematics, SFU  Radoicic's Conjecture and Tight Graphs 
Tue 29 Oct / 2:00 / EAA1100  Bruce Reed, McGill University  Tree Width and Excluded Minors 
Tue 05 Nov / 2:00 / EAA1100  Richard Anstee, Mathematics, U. British Columbia  Forbidden Configurations 
Sat 09 Nov / 116 / UVic  A. Proskurowski, B. Grunbaum, J. Siran  Combinatorial Potlatch at UVic 
Tue 12 Nov / 2:00 / EAA1100  Qianping Gu, Comp. Sci., SFU  A 1.8Approximation Algorithm for Embedding Hypergraphs in a Cycle 
Tue 19 Nov / 2:00 / EAA1100  Daniel Kral, Charles University  Channel Assignment Problem 
Thu (!) 21 Nov / 2:00 / EAA1100  Vadim V. Lozin, Rutgers  Boundary Classes of Graphs for NPhard Problems 
Tue 26 Nov / 2:00 / EAA1100  Ron Ferguson, PIMS/SFU  Polyphase Sequences with Low Autocorrelation 
Tue 03 Dec / 2:00 / EAA1100  Cynthia Loten , Mathematics, SFU  Retractions and a Generalization of Chordal Graphs 
Summer 2002 

Tue 14 May / 3:30 / EAA1100  Rados Rodoicic, MIT  Rainbow Arithmetic Progressions 
Tue 28 May / 3:30 / EAA1100  Sean McGuinness, U. Umea and Montana  Circuits intersecting cocircuits in graphs and matroids. 
Tue 11 Jun / 3:30 / EAA1100  Claude Tardif, U. Regina  The projectivity of the graphs $G_d^k$ 
Tue 18 Jun / 3:30 / K9509  Joergen BangJensen, U. Southern Denmark  Steiner type problems in tournamentlike digraphs 
Tue 25 Jun / 3:30 / K 9500 (!)  Winfried Hochstaettler, Brandenburg U. of Tech.  Maxflow mincut duality for a paint shop problem 
Tue 02 Jul / 3:30 / EAA1100  Luis Goddyn, Mathematics, SFU  Flows and Tensions on maps of high edge width. 
Tue 09 Jul / 3:30 / EAA1100  Peter Higgins, Mathematics,  Sur une Th\'eor\`eme de Sierpi\'nsk (In English!) 
Tue 16 Jul / 3:30 / EAA1100  KheeMeng Koh, Nat. U. of Singapore  On the Chromaticity of graphs 
Tue 23 Jul / 3:30 / EAA1100  UBC PiMS Event  Lovasz  See their program for titles and abstracts. 
Tue 30 Jul / 3:30 / EAA1100  Marnie Mishna  Enumeration via Dfinite Symmetric Functions 
Tue 6 Aug / 3:30 / EAA1100  Luis Goddyn  Chromatic numbers of matroids 
Tue 6 Aug / 3:30 / EAA1100  Bill McCuaig  Edmonds' arcdisjoint arborescences theorem 
Spring 2002 

Tue 05 Feb / 3:30 / EAA1100  Reza Naserasr, Math, SFU  Homomorphisms from sparse graphs with large girth. 
Tue 12 Feb / 3:30 / EAA1100  Qianping Gu, CS, SFU  Permutation routing on WDM optical hypercubes I. 
Tue 19 Feb / 3:30 / EAA1100  Qianping Gu, CS, SFU  Permutation routing on WDM optical hypercubes II. 
Tue 05 Mar / 3:30 / EAA1100  Luis Goddyn, Math, SFU  Graph coloring and Matroids. 
Tue 05 Mar / 3:30 / EAA1100  Tom Friedetzky, CS, SFU  The Natural WorkStealing Algorithm is Stable 
Tue 19 Mar / 3:30 / EAA1100  Petr Lisonek, Math, SFU  Caps and Binary Linear Codes 
Tue 26 Mar / 3:30 / EAA1100  Petra Berenbrink, CS, SFU  Simple routing strategies for Adversarial networks. 
Tue 02 Apr / 3:30 / EAA1100  Pavol Hell, CS/Math, SFU  Complexity of generalized listcolourings. 
Tue 09 Apr / 3:30 / EAA1100  Laco Stacho, Math, SFU  Edgedisjoint spanners in the Cartesian product of graphs 
Fall 2001  
Tue 18 Sep / 3:30 / EAA1100  Luis Goddyn, Math, SFU  Binary Gray Codes with long bit runs. 
Tue 25 Sep / 3:30 / EAA1100  Louis Ibarra, CS, SFU  The cliqueseparator graph for chordal graphs and its subclasses. 
Tue 02 Oct / 3:30 / EAA1100  Ladislav Stacho, Math, SFU  New upperbound for chromatic number. 
Tue 09 Oct / 3:30 / EAA1100  Amitava Bhattacharya, TIFR  A short proof of a EKR type theorem. 
Tue 16 Oct / 3:30 / EAA1100  Juan Montellano, UNAM  AntiRamsey Problems. 
Tue 23 Oct / 3:30 / EAA1100  Ladislav Stacho, Math, SFU  Spannin trees with bounded number of branch vertices. 
Tue 30 Oct / 3:30 / EAA1100  Bill McCuaig, Math, SFU  The Hat Game. 
Tue 06 Nov / 3:30 / EAA1100  Mohammad Ghebleh, Math, SFU  Uniuqely list colorable graphs. 
Tue 13 Nov / 3:30 / EAA1100  Petr Lisonek, Math, SFU  Geometric Representations of Graphs. 
Tue 20 Nov / 3:30 / EAA1100  Luis Goddyn, Math, SFU  Using the Nullstellensatz for edge list colouring. 
Tue 27 Nov / 3:30 / EAA1100  Reza Naserasr, Math, SFU  On the covering number of graphs. 
Tue 04 Dec / 3:30 / EAA1100  Roded Sharan, Technion, Haifa  Incomplete Directed Perfect Phylogeny. 
Tue 11 Dec / 3:30 / EAA1100  Frederic Havet, CNRS, France  Design of fault tolerant onboard networks with priorities 
Tue 18 Dec / 3:30 / EAA1100  Mirka Miller, Newcastle, Australia  Eccentric Digraphs 
Summer 2001  
Thu 10 May / 3:30 / EAA1100  John Mighton, Fields Inst.  A New Reduction of Graph Theory and Binary Matroids 
Tue 15 May / 3:30 / EAA1100  Sergei Bespamyatnikh, CS, U. British Columbia  Graph of triangulations of convex polytopes in R^3 
Tue 26 Jun / 3:30 / K9509  Matt DeVos, Princeton U.  On Packing TJoins 
Tue 10 Jul / 3:30 / EAA1100  Moishe Rosenfeld, U. of Washington  Hamiltonian Decomposition of Prisms over cubic graphs 
Tue 17 Jul / 3:30 / K9509  Claude Tardif, U. of Regina  Fractional aspects of Hedetniemi's conjecture 
Spring 2001  
Tue 30 Jan / 3:30 / K9509  Reza Naserasr, Math, SFU  On $\Delta$critical graphs 
Wed 31 Jan / 2:30 / K9509  Brett Stevens, Math, SFU  Packing and covering designs 
Tue 06 Feb / 3:30 / K9509  Luis Goddyn, Math, SFU  Rangerestricted flows in regular matroids. 
Tue 13 Feb / 3:30 / EAA1100  Jarik Nesetril, Charles University, Prague  On a homomorphism duality for oriented trees. 
Tue 20 Feb / 2:30 / EAA1100  Valentine Kabanets, Inst. for Advanced Study  In search of an easy witness: Exponential time vs probabilistic polytime. 
Tue 20 Feb / 3:40 / EAA1100  Wayne Broughton, Okanagan University Coll.  Quasi3 Designs and Hadamard Matrices. 
Wed 28 Feb / 3:30 / K9509  Pavol Hell, SFU  List homomorphisms and partitions 
Tue 6 Mar / 3:30 / EAA1100  Steph Durocher, U. British Columbia  The tangled tale of the rectilinear crossing number of K_n 
Tue 13 Mar / 3:30 / EAA1100  Richard Anstee, U. British Columbia  Small Forbidden Configurations 
Tue 20 Mar / 3:30 / EAA1100  Brett Stevens, Math, SFU  Packing Arrays II: Beyond Codes, the Triumphant Return of Designs! 
Tue 27 Mar / 2:30 / EAA1100  Lou Ibarra, U. Victoria  Dynamic algorithms for chordal and interval graphs. 
Tue 27 Mar / 3:30 / EAA1100  Jonathan Jedwab, HewlettPackard Labs, Bristol  Combinatorial Design Theory in an Industrial Research Lab 
Wed 28 Mar / 2:30 / K9509  William Evans, U. British Columbia  Recovering lines with fixed linear probes 
Wed 28 Mar / 3:30 / K9509  Petr Lisonek, Math, SFU  Classification of Binary Linear Codes with Small Codimension 
Tue 3 Apr / 3:30 / EAA1100  Mateja Sajna, Capilano Coll.  Decomposition of complete graphs into cycles of a fixed length 
Wed 4 Apr / 3:30 / K9509  Veselin Jungic, Math, SFU  Diophantine Ramsey Theory and Recurrence in Dynamical Systems 
Tue 10 Apr / 3:30 / EAA1100  Ladislav Stacho, Math, SFU  Edge Disjoint Cycles Through Prescribed Vertices 
Tue 17 Apr / 3:30 / EAA1100  Nancy Neudauer, Pacific Lutheren U.  Bicircular Matroids and Transversal Presentations 