EN
Let $f = ax + bx^q + x^{2q-1} ∈ 𝔽_q[x]$. We find explicit conditions on a and b that are necessary and sufficient for f to be a permutation polynomial of $𝔽_{q²}$. This result allows us to solve a related problem: Let $g_{n,q} ∈ 𝔽_p[x]$ (n ≥ 0, $p = char 𝔽_q$) be the polynomial defined by the functional equation $∑_{c∈ 𝔽_q} (x+c)^n = g_{n,q} (x^q -x)$. We determine all n of the form $n = q^α - q^β - 1$, α > β ≥ 0, for which $g_{n,q}$ is a permutation polynomial of $𝔽_{q²}$.