Critical and injective modules over skew polynomial rings

Let $R$ be a commutative local $k$-algebra of Krull dimension one, where $k$ is a field. Let $α$ be a $k$-algebra automorphism of $R$, and define $S$ to be the skew polynomial algebra $R[θ; α]$. We offer, under some additional assumptions on $R$, a criterion for $S$ to have injective hulls of all simple $S$-modules locally Artinian - that is, for $S$ to satisfy property $(\diamond)$. It is easy and well known that if $α$ is of finite order, then $S$ has this property, but in order to get the criterion when $α$ has infinite order we found it necessary to classify all cyclic (Krull) critical $S$-modules in this case, a result which may be of independent interest. With the help of the above we show that $\hat{S}=k[[X]][θ, α]$ satisfies $(\diamond)$ for all $k$-algebra automorphisms $α$ of $k[[X]]$.

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