On Unit Groups of Completely Primary Finite Rings

Abstract

Let R be a commutative completely primary finite ring with the unique maximal ideal J such that J3 = (0) and J2 6= (0): Then R⁄J ∼= GF(pr) and the characteristic of R is pk, where 1 ≤ k ≤ 3, for some prime p and positive integers k, r. Let Ro = GR (pkr,pk) be a galois subring of R so that R = Ro ⊕ U ⊕ V ⊕ W, where U, V and W are finitely generated Ro-modules. Let non-negative integers s, t and be numbers of elements in the generating sets for U, V and W, respectively. In this work, we determine the structure of the subgroup 1+W of the unit group R* in general, and the structure of the unit group R* of R when s = 3, t = 1; ≥ 1 and characteristic of R is p. We then generalize the solution of the cases when s = 2, t = 1; t = s(s +1)⁄2 for a fixed s; for all the characteristics of R ; and when s = 2, t = 2, and characteristic of R is p to the case when the annihilator ann(J ) = J2 + W, so that ≥ 1. This complements the author’s earlier solution of the problem in the case when the annihilator of the radical coincides with the square of the radical.