Good morning,

I am trying to run a regression for a waste Kuznets curve (panel data). I used the quadratic term of GDP as suggested by most authors but this variable is then omitted because of collinearity. I understand that the multicollinearity comes from its relationship with the GDP variable, but do not understand how other authors fixed this problem.

Would anyone be able to find a solution?

Code:
gen GDP2t = GDP * GDP
gen lnGDP2t = ln(GDP2t)
xtreg lnWaste lnGDP lnGDP2t lnUrban lnHhsize lnEducated