Thus, the second partial derivative test indicates that "f"("x", "y") has saddle points at (0, −1) and (1, −1) and has a local maximum at formula_9 since formula_13. At the remaining critical point (0, 0) the second derivative test is insufficient, and one must use higher order tests or other tools to determine the behavior of the function at this point. (In fact, one can show that "f" takes both positive and negative values in small neighborhoods around (0, 0) and so this point is a saddle point of "f".)