Abstract
The aim of this article is to prove the irreducibility of the polynomial Lambda(Y) = Y(d) + lambda(d-1)Y(d-1) + ... + lambda(0) over F(q)[X] where lambda(i) is an element of F(q)[X] and deg lambda(d-1) > deg lambda(i) for each i not equal d - 1. We discuss in particular connections between the irreducible polynomials Lambda and the number of Pisot elements in the case of formal power series.