Abstract. Let V be an absolutely irreducible representation of a profinite group G over the residue field k of a noetherian local ring O. For local complete O-algebras A with residue field k the representations of G over A that reduce to V over k are given by O-algebra homomorphisms R → A, where R is the universal deformation ring of V . We show this with an explicit construction of R. The ring R is noetherian if and only if H 1 (G, End k (V )) has finite dimension over k.
IntroductionLet G be a profinite group and let k be a field. By a k-representation of G we mean a finite dimensional vector space over k with the discrete topology, equipped with a continuous k-linear action of G. If V is a k-representation of G and A is a complete local ring with residue field k, then a deformation of V in A is an isomorphism class of continuous representations of G over A that reduce to V modulo the maximal ideal of A; precise definitions are given in Section 2. We denote by Def(V, A) the set of such deformations.Let V be an absolutely irreducible k-representation of G. The object of this chapter is to give a straight-forward construction of a ring R, the universal deformation ring, which represents the functor Def(V, −). In a purely algebraic setting, without considerations of continuity, a similar construction was already given by Procesi in the seventies [9, Chap. IV, Lemma 1.7; 10]. The existence of R in the present context was deduced first by Mazur [8] with Schlessinger's criteria for pro-representability [12]. An alternative construction was given recently by Faltings (see [5] and Section 7 below).The main result of this chapter, formulated below as Theorem (2.3), is actually a little more general than Mazur's. Following Schlessinger, Mazur works only with noetherian rings, and this forces him to assume at the outset that a certain cohomology group is finite. For our argument, the noetherian condition is a hindrance, and we find it more convenient to follow Grothendieck [6] and work with not necessarily noetherian rings that are projective limits of artinian rings. This allows us to drop Mazur's cohomological condition; it reappears only at the end, as a necessary and sufficient condition for R to be noetherian.