Failure of the Only-Critical-Point-in-Town Test

Here is  a polynomial example of the failure of the only-critical-point-in-town test. There is a local minimum, it is the only critical point, but it is not an absolute minimum. This function, found by Calvert and Vamanamurthy, is simply z = x^2 (1 + y)^3 + 7 y^2.

Created by Wolfram Mathematica 6.0  (28 October 2007)