La factorisation de $x^{p^l}-x\in{\mathbb F}\_p[x]$ selon la trace
  We present a few factorizations of polynomials over finite fields. These factorizations are related to traces, compositions of polynomials and binomial coefficients. As a corollary we obtain a description of all irreducible polynomials $Q\in {\mathbb F}\_2[x]$ such that $Q(1+x+x^2)$ (or $Q(x+x^2)$) remain irreducible. 
Roland Bacher (IF)