braninmodif <- function(xx, a=1, b=5.1/(4*pi^2), c=5/pi, r=6, s=10, t=1/(8*pi))
{
  ##########################################################################
  #
  # BRANIN FUNCTION, MODIFIED
  #
  # Authors: Sonja Surjanovic, Simon Fraser University
  #          Derek Bingham, Simon Fraser University
  # Questions/Comments: Please email Derek Bingham at dbingham@stat.sfu.ca.
  #
  # Copyright 2013. Derek Bingham, Simon Fraser University.
  #
  # THERE IS NO WARRANTY, EXPRESS OR IMPLIED. WE DO NOT ASSUME ANY LIABILITY
  # FOR THE USE OF THIS SOFTWARE.  If software is modified to produce
  # derivative works, such modified software should be clearly marked.
  # Additionally, this program is free software; you can redistribute it 
  # and/or modify it under the terms of the GNU General Public License as 
  # published by the Free Software Foundation; version 2.0 of the License. 
  # Accordingly, this program is distributed in the hope that it will be 
  # useful, but WITHOUT ANY WARRANTY; without even the implied warranty 
  # of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 
  # General Public License for more details.
  #
  # For function details and reference information, see:
  # http://www.sfu.ca/~ssurjano/
  #
  ##########################################################################
  #
  # INPUTS:
  #
  # xx = c(x1, x2)
  # a = constant (optional), with default value 1
  # b = constant (optional), with default value 5.1/(4*pi^2)
  # c = constant (optional), with default value 5/pi
  # r = constant (optional), with default value 6
  # s = constant (optional), with default value 10
  # t = constant (optional), with default value 1/(8*pi)
  #
  ##########################################################################
  
  x1 <- xx[1]
  x2 <- xx[2]
  
  term1 <- a * (x2 - b*x1^2 + c*x1 - r)^2
  term2 <- s*(1-t)*cos(x1)
  
  y <- term1 + term2 + s + 5*x1
  return(y)
}