function S=test6(x)
% Wood (with several local minima)
% x0 = [-3, -1, -3, -1]'
% xo = [1, 1, 1, 1]'
% S(xo) = 0

S=100*(x(2)-x(1).^2).^2+(1-x(1)).^2+90*(x(4)-x(3).^2).^2+(1-x(3)).^4+ ...
  10.1*(x(2)-1).^2+(x(4)-1).^2+19.8*((x(2)-1).*(x(4)-1)).^2;
