Suppose that (x, y)=(a, b) is a critical point of f(x, y)
and D(x, y)=∂2f∂ x2 ∂2f∂ y2 −(∂2f∂ x∂ y )2. Then,
If D(a, b) > 0 and ∂2f∂ x2 <0 then the point (a, b) is a local maximum.
If D(a, b) > 0 and ∂2f∂ x2 >0 then the point (a, b) is a local minimum.
If D(a, b)<0 then the point (a, b) is a saddle point.
If D(a, b)=0 then test failed and the point (a, b) can be a local maximum, local minimum, saddle or neither.