A Categorical Approach to Finiteness Conditions

Abstract

We introduce a general categorical framework for finiteness conditions that unifies classical notions such as Noetherianness, Artinianness, and various forms of topological compactness. This is achieved through the concept of \textbf{$τ$-compactness}, defined relative to a \textbf{coverage} $τ$. A coverage on a category $C$ is a specified class of covering diagrams, which are functors $F\colon I \to C/c$ of a specified variance, where the indexing category $I$ is equipped with a set of 'designated small objects'. An object $c$ is $τ$-compact if every such covering diagram over it stabilizes at some designated small object. As we permit functors of mixed variance, our framework simultaneously models ascending chain conditions (such as Noetherianness) and descending chain conditions (such as Artinianness, topological compactness via closed sets). The role of protomodularity of the ambient category emerges as a crucial property for proving strong closure results. Under suitable compatibility assumptions on the coverage, we show that in a protomodular category, the class of $τ$-compact objects is closed under quotients and extensions. In a pointed context, this implies closure under finite products, generalizing the classical theorem that a finitely generated module over a Noetherian ring is itself Noetherian. We also leverage protomodularity to establish a categorical Hopfian property for Noetherian objects. Our main application shows that in any regular protomodular category with an initial object, the classes of Noetherian and Artinian objects are closed under subobjects, regular quotients, and extensions. As a consequence, in any abelian category, these classes of objects form exact subcategories.