The equation for the function z = f(x,y) to create this surface is:[br][br][math]z=-0.15x^2-0.25y^2+1[/math].[br][br]As can be seen by looking at the gradient in the x and the y directions, and the fact that [math]\frac{\partial^2z}{\partial x\partial y}=0[/math], the point at the base is a local minimum.