Gradable modules over artinian rings

Abstract

Let $\Lambda$ be a $\mathbb{Z}$-graded artin algebra. Two classical results of Gordon and Green state that if $\Lambda$ has only finitely many indecomposable gradable modules, up to isomorphism, then $\Lambda$ has finite representation type, and if $\Lambda$ has finite representation type then every $\Lambda$-module is gradable. We generalize these results to $\mathbb{Z}$-graded right artinian rings $R$. The key tool is a characterization of gradable modules: a f.g. right $R$-module is gradable if and only if its "pull-up" is pure-projective. Using this we show that if there is a bound on the graded-lengths of f.g. indecomposable graded $R$-modules, then every f.g. $R$-module is gradable. As another consequence, we see that if a graded artin algebra has an ungradable module, then it has a Pr\"ufer module which is not of finite type, and hence it has a generic module by work of Ringel