%gradient with respect to x of limit function g
function y = gradgx(x,u)
x = x(:);
u = u(:);
v(1) = 400*u(2) + 2000;
v(2) = 250*u(3) + 1000;
v(3) = 100*u(1) + 500;
v(4) = exp(0.7386 + 0.0998*u(4));
%fix the rest!
grad(1) = 4*v(1)/(x(1)^2*x(2)^2*v(4))...
           + 8*v(2)/(x(1)^3*x(2)*v(4))...
           + 2*v(3)^2/(x(1)^3*(x(2)*v(4))^2);
grad(2) = 8*v(1)/(x(1)*x(2)^3*v(4))...
           + 4*v(2)/((x(1)*x(2))^2*v(4))...
           + 2*v(3)^2/(x(2)^3*(x(1)*v(4))^2);
grad = grad(:);
y = -grad;