we know that we use the following formula to find a turning point in income inequality researches:
Gini=B0 + B1gdp + B2gdp^2 + e
u = exp(-B1/2B2)
Gini=B0 + B1gdp + B2gdp^2 + B3gdp^3 + e;
For the cubic equation there will be two turning points as the derivative will be a quadratic equation.
this is the answer if you look for an answer for that equation