%x=-2:.1:2; y=x;
x=-4:.1:4; y=x;
n=size(x,2);
a=[];
axis('xy')

for j=1:n;
   for k=1:n;
     a(j,k)=F(x(j),y(k));
   end
end
mesh(x,y,a)
figure
contour(a)