Categorical Complexity

TL;DR

This paper introduces a new categorical notion of complexity for diagrams, objects, and functors, connecting classical computational complexity with category theory across diverse mathematical contexts.

Contribution

It defines categorical complexity and demonstrates its relevance to classical complexity notions, offering a natural mathematical framework for complexity class separation.

Findings

Categorical complexity recovers classical circuit complexity.

The framework applies to various mathematical categories.

Functor complexity may parallel classical complexity class separation.

Abstract

We introduce a notion of complexity of diagrams (and in particular of objects and morphisms) in an arbitrary category, as well as a notion of complexity of functors between categories equipped with complexity functions. We discuss several examples of this new definition in categories of wide common interest, such as finite sets, Boolean functions, topological spaces, vector spaces, semi-linear and semi-algebraic sets, graded algebras, affine and projective varieties and schemes, and modules over polynomial rings. We show that on one hand categorical complexity recovers in several settings classical notions of non-uniform computational complexity (such as circuit complexity), while on the other hand it has features which make it mathematically more natural. We also postulate that studying functor complexity is the categorical analog of classical questions in complexity theory about separating different complexity classes.