Visual zero gradient

Here's the graph of the scalar field $f(x, y) = -x^3 + \dfrac{y^2}{3} + \dfrac{x}{2}$f, left parenthesis, x, comma, y, right parenthesis, equals, minus, x, cubed, plus, start fraction, y, squared, divided by, 3, end fraction, plus, start fraction, x, divided by, 2, end fraction, where $-1 < x < 1$minus, 1, is less than, x, is less than, 1 and $-1 < y < 1$minus, 1, is less than, y, is less than, 1.