function [y] = welchetal92(xx)

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
% WELCH ET AL. (1992) 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:
% 
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
% INPUT:
%
% xx = [x1, x2, ..., x20]
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

x1  = xx(1);
x2  = xx(2);
x3  = xx(3);
x4  = xx(4);
x5  = xx(5);
x6  = xx(6);
x7  = xx(7);
x8  = xx(8);
x9  = xx(9);
x10 = xx(10);
x11 = xx(11);
x12 = xx(12);
x13 = xx(13);
x14 = xx(14);
x15 = xx(15);
x16 = xx(16);
x17 = xx(17);
x18 = xx(18);
x19 = xx(19);
x20 = xx(20);

term1 = 5*x12 / (1+x1);
term2 = 5 * (x4-x20)^2;
term3 = x5 + 40*x19^3 - 5*x19;
term4 = 0.05*x2 + 0.08*x3 - 0.03*x6;
term5 = 0.03*x7 - 0.09*x9 - 0.01*x10;
term6 = -0.07*x11 + 0.25*x13^2 - 0.04*x14;
term7 = 0.06*x15 - 0.01*x17 - 0.03*x18;

y = term1 + term2 + term3 + term4 + term5 + term6 + term7;

end