function y = PrPf(x)
% Matlab Code by A. Hedar (Nov. 23, 2005).
y = 0.6224*x(1)*x(3)*x(4)+1.7781*x(2)*x(3)^2+3.1661*x(1)^2*x(4)+19.84*x(1)^2*x(3);