# Irreducible and separable polynomial

Let $f(x)$ be an irreducible polynomial over $F[X]$, with $char(F)=p$ prime number. We know that $\exists t \in \mathbb{N} | f(x)=g((x^p)^t)$. We shall prove that $g(x)$ is irreducible and separable.

I know a Proposition which states that if $f(x)$ is irreducible it is equivalent to be in the form $f(x)=h(x^p)$ with $char(F)=p\ne 0$ and have multiple roots. But I can't figure out how to fruitfully apply it.