  Using recent results on solving the equation $X^{2^k+1}+X+a=0$ over a finite
field $\mathbb{F}_{2^n}$, we address an open question raised by the first
author in WAIFI 2014 concerning the APN-ness of the Kasami functions $x\mapsto
x^{2^{2k}-2^k+1}$ with $gcd(k,n)=1$, $x\in\mathbb{F}_{2^n}$.
