El gradiente cero visualmente

Esta es la gráfica del campo escalar $f(x, y) = -x^3 + \dfrac{y^2}{3} + \dfrac{x}{2}$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, donde $-1 < x < 1$minus, 1, is less than, x, is less than, 1 y $-1 < y < 1$minus, 1, is less than, y, is less than, 1.