Moore-Penrose Dagger Categories

TL;DR

This paper extends the concept of Moore-Penrose inverses to dagger categories, introducing Moore-Penrose dagger categories and characterizing maps with M-P inverses using generalized decompositions.

Contribution

It introduces Moore-Penrose dagger categories, provides multiple examples, and establishes equivalences between M-P inverses and generalized decompositions in dagger categories.

Findings

Defined Moore-Penrose dagger categories.

Established equivalence between M-P inverses and generalized decompositions.

Characterized maps with M-P inverses in various dagger categories.

Abstract

The notion of a Moore-Penrose inverse (M-P inverse) was introduced by Moore in 1920 and rediscovered by Penrose in 1955. The M-P inverse of a complex matrix is a special type of inverse which is unique, always exists, and can be computed using singular value decomposition. In a series of papers in the 1980s, Puystjens and Robinson studied M-P inverses more abstractly in the context of dagger categories. Despite the fact that dagger categories are now a fundamental notion in categorical quantum mechanics, the notion of a M-P inverse has not (to our knowledge) been revisited since their work. One purpose of this paper is, thus, to renew the study of M-P inverses in dagger categories. Here we introduce the notion of a Moore-Penrose dagger category and provide many examples including complex matrices, finite Hilbert spaces, dagger groupoids, and inverse categories. We also introduce generalized versions of singular value decomposition, compact singular value decomposition, and polar decomposition for maps in a dagger category, and show how, having such a decomposition is equivalent to having M-P inverses. This allows us to provide precise characterizations of which maps have M-P inverses in a dagger idempotent complete category, a dagger kernel category with dagger biproducts (and negatives), and a dagger category with unique square roots.