Generalized Rank via Minimal Subposet

Abstract

Let $\mathcal{C}$ be a small, connected category with finite hom-sets. We show that if the embedding of a connected subcategory $\mathcal{J}$ is both initial and final, then the restriction of any $\mathcal{C}$-module along $\mathcal{J}$ preserves the generalized rank-or equivalently, the multiplicity of the ``entire" interval modules for $\mathcal{C}$ and $\mathcal{J}$. Conversely, we prove that this property characterizes initial and final embeddings when both $\mathcal{C}$ and $\mathcal{J}$ are posets satisfying certain mild constraints and the embedding is full. For $\mathcal{C}$ a poset under these conditions, we describe the minimal full subposet whose embedding is initial or final. This generalizes an observation made by Dey and Lesnick. We also extend a result of Kinser on the generalized rank invariant to small categories.