“…These studies are seen to have investigated the validity of the inverse-U relationship using the variables of CO 2 emissions, per capita income, and the per capita income squared. Another model for the validity of the environmental Kuznets curve involves studies that have investigated the validity of the cubic form (Şahinöz & Fotourehchi, 2013;Allard et al, 2018;Manga & Cengiz, 2020;Özdemir & Koç, 2020;Güzel, 2021).…”