Ramification des séries formelles

Abstract Let p be a prime number. Let k be a finite field of characteristic p. The subset X + X2k[[X]] of the ring k[[X]] is a group under the substitution law ○ sometimes called the Nottingham group of k, it is denoted by Rk. The ramification of one series γ ∈ Rk is caracterized by its lower ramification numbers: , as well as its upper ramification numbers: By Sen's theorem, the um(γ) are integers. In this paper, we determine the sequences of integers (um) for which there exists γ ∈ Rk such that um(γ) = um for all integer m & 0.