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 ($\sigma, \tau$) 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 $(\sigma, \tau)$-derivations of the group ring $RG$. First, we develop a necessary and sufficient condition for a $(\sigma, \tau)$-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 $(\sigma, \tau)$-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 $(\sigma, \tau)$-derivations of $RG$ inner? Or is the space of outer $(\sigma, \tau)$-derivations trivial? More precisely, we give a sufficient condition under which all $(\sigma, \tau)$-derivations of $RG$ are inner and a sufficient condition under which $RG$ has non-trivial outer $(\sigma, \tau)$-derivations. Our result helps in generating several examples of non-trivial outer derivations.