Subcategories of Module Categories via Restricted Yoneda Embeddings

We propose a framework for producing interesting subcategories of the category ${}_A\mathsf{M}\mathsf{o}\mathsf{d}$ of left $A$-modules, where $A$ is an associative algebra over a field $k$. The construction is based on the composition, $Y$, of the Yoneda embedding of ${}_A\mathsf{M}\mathsf{o}\mathsf{d}$ with a restriction to certain subcategories $\mathcal{B}\subset {}_A\mathsf{M}\mathsf{o}\mathsf{d}$, typically consisting of cyclic modules. We describe the subcategories on which $Y$ provides an equivalence of categories. This also provides a way to understand the subcategories of ${}_A\mathsf{M}\mathsf{o}\mathsf{d}$ that arise this way. Many well-known categories are obtained in this way, including categories of weight modules and Harish-Chandra modules with respect to a subalgebra $\mathrm{\Gamma }$ of $A$. In other special cases the equivalence involves modules over the Mickelsson step algebra associated to a reductive pair of Lie algebras.