min f(z) = (4 - 2.1*x^2 + x^4/3)*x^2 + x*y + (-4 + 4*y^2)*y^2; z=(x, y)
x = -0.0896 y = 0.7130 z = (x, y) f(z) = -1.0316
x = 0.0896 y = -0.7130 z = (x, y) f(z) = -1.0316