If f (x, y) is of class C2, has a critical point at (a, b), and fx x (a, b) f yy (a, b)
Chapter 4, Problem 15(choose chapter or problem)
If f (x, y) is of class C2, has a critical point at (a, b), and fx x (a, b) f yy (a, b) fx y (a, b) 2 < 0, then f has a saddle point at (a, b).
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