goldpr <- function(xx)
{
  ##########################################################################
  #
  # GOLDSTEIN-PRICE FUNCTION
  #
  # 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/
  #
  ##########################################################################
  #
  # INPUT:
  #
  # xx = c(x1, x2)
  #
  ##########################################################################
  
  x1 <- xx[1]
  x2 <- xx[2]
	
  fact1a <- (x1 + x2 + 1)^2
  fact1b <- 19 - 14*x1 + 3*x1^2 - 14*x2 + 6*x1*x2 + 3*x2^2
  fact1 <- 1 + fact1a*fact1b
	
  fact2a <- (2*x1 - 3*x2)^2
  fact2b <- 18 - 32*x1 + 12*x1^2 + 48*x2 - 36*x1*x2 + 27*x2^2
  fact2 <- 30 + fact2a*fact2b
	
  y <- fact1*fact2
  return(y)
}