Functors induced by comma categories
Suddhasattwa Das
TL;DR
The paper develops a canonical functorial bridge between comma-category objects and left-slice categories. By studying L-shaped universal squares and introducing the Lim-Pre construction, it shows how each morphism in a comma arrangement induces a functor between left slices, with the construction being functorial in morphisms and compositional under composition. The main contributions include the Dyn construction, two key theorems establishing induced functors and their compositionality, and a detailed diagrammatic framework connecting comma, arrow, and slice categories. This provides a universal, diagrammatic way to interpret dynamics and substructure across disparate categories, with potential applications to dynamical systems, subobject classifiers, and categorical representations of orbits and reachability. The results unify several known structures (e.g., arrow categories, subobject lattices) under a general, universal principle for leveraging comma-category data to generate slice-wise functors.
Abstract
Category theory provides a collective description of many arrangements in mathematics, such as topological spaces, Banach spaces and game theory. Within this collective description, the perspective from any individual member of the collection is provided by its associated left or right slice. The assignment of slices to objects extends to a functor from the base category, into the category of categories. Slice categories are a special case of the more general notion of comma categories. Comma categories are created when two categories $\mathcal{A}$ and $\mathcal{B}$ transform into a common third category $\mathcal{C}$, via functors $F,G$. Such arrangements denoted as $\Comma{F}{G}$ abound in mathematics, and provide a categorical interpretation of many constructions in Mathematics. Objects in this category are morphisms between objects of $\mathcal{A}$ and $\mathcal{B}$, via the functors $F,G$. We show that these objects also have a natural interpretation as functors between slice categories of $\mathcal{A}$ and $\mathcal{B}$. Thus even though $\mathcal{A}$ and $\mathcal{B}$ may have completely disparate structures, some morphisms in $\mathcal{C}$ lead to functors between their respective slices. We present this relation in the form of a functor from $\mathcal{C}$ into the category of left slices. The proof of our main result requires a deeper look into associated categories, in which the objects themselves are various commuting diagrams.