Homomorphic encoders of profinite abelian groups I

Abstract

Let {Gi|i∈N} be a family of finite Abelian groups. We say that a subgroup G≤∏i∈NGi is order controllable if for every i∈N there is ni∈N such that for each c∈G, there exists a∈G satisfying that a|[1,i]=c|[1,i], supp(a)⊆[1,ni], and order(a) divides order(c|[1,ni]). In this paper we investigate the structure of order controllable subgroups. It is proved that every order controllable, profinite, abelian group contains a subset {gn|n∈N} that topologically generates the group and whose elements gn all have finite support. As a consequence, sufficient conditions are obtained that allow us to encode, by means of a topological group isomorphism, order controllable profinite abelian groups. Further applications of these results to group codes will appear subsequently.