Let $q$ be a power of prime $p$ and $\zeta_p$ be the complex $p$ th root of unity. We denote by $\mathbb{F}_q$ the finite field of $q$ elements and by $Tr$ the absolute trace function $\mathbb{F}_q\rightarrow \mathbb{F}_p$ . The Kloosterman sum at a point $a \in \mathbb{F}_q$ is defined by the equation

$\hspace{3cm} K_{q}(a)=\displaystyle\sum_{x\in\mathbb{F}_q^*}\psi(x+ax^{-1})$

where $\psi:\mathbb{F}_q\rightarrow \mathbb{Q}(\zeta_p)$ be the canonical additive character of $\mathbb{F}_q$ defined by $\psi(x)=\zeta_p^{Tr(x)}.$

It is known that $K_{q}(a)\in\mathbb{Z}$ for $p=2,3$.

Is $K_{q}(a)\in\mathbb{Z}$ for other primes ?