In Exercises, both first partial derivatives of the function f (x, y) are zero at the given points. Use the second-derivative test to determine the nature of f (x, y) at each of these points. If the second-derivative test is inconclusive, so state.

f (x, y) = x⁴ - 4xy + y⁴; (0, 0), (1, 1), (-1, -1)