Inner and Outer Twisted Derivations of Cyclic Group Rings

Abstract

In this article, we study twisted derivations of cyclic group rings. Let $R$ be a commutative ring with unity, $G$ be a finite cyclic group, and ($σ, τ$) be a pair of $R$-algebra endomorphisms of the group algebra $RG$, which are $R$-linear extensions of the group endomorphisms of $G$. In this article, we give two characterizations concerning $(σ, τ)$-derivations of the group ring $RG$. First, we develop a necessary and sufficient condition for a $(σ, τ)$-derivation of $RG$ to be inner. Second, we provide a necessary and sufficient condition for an $R$-linear map $D: RG \rightarrow RG$ with $D(1) = 0$ to be a $(σ, τ)$-derivation. We also illustrate our theorems with the help of examples. As a consequence of these two characterizations, we answer the well-known twisted derivation problem for $RG$: Under what conditions are all $(σ, τ)$-derivations of $RG$ inner? Or is the space of outer $(σ, τ)$-derivations trivial? More precisely, we give a sufficient condition under which all $(σ, τ)$-derivations of $RG$ are inner and a sufficient condition under which $RG$ has non-trivial outer $(σ, τ)$-derivations. Our result helps in generating several examples of non-trivial outer derivations.

Theorems & Definitions (43)

• Theorem 1.1: Chaudhuri2019
• Theorem 3.5
• Theorem 4.4
• Definition 2.1
• Definition 2.2
• Remark 2.3
• Lemma 2.4
• proof
• Remark 2.5
• Definition 2.6