Collinear triples in permutations

Let $\alpha :{\mathbb{F}}_q\to {\mathbb{F}}_q$ be a permutation and $\mathrm{\Psi }(\alpha )$ be the number of collinear triples in the graph of $\alpha$, where ${\mathbb{F}}_q$ denotes a finite field of $q$ elements. When $q$ is odd Cooper and Solymosi once proved $\mathrm{\Psi }(\alpha )\ge (q-1)\mathrm{/}4$ and conjectured the sharp bound should be $\mathrm{\Psi }(\alpha )\ge (q-1)\mathrm{/}2$. In this note we indicate that the Cooper-Solymosi conjecture is true.