Informally a doubly indexed category is a unitary lax double functor , where the latter is the proarrow equipment of categories, functors, profunctors and natural transformations.
Formally, it is a right -module, i.e. it’s a right action of the double category .
With this name, they were defined by David Jaz Myers in his book on categorical system theory.
Let be a double category, presented as a pseudomonad in . A doubly indexed category over is a right -module, meaning it consists of a functor together with a map over , satisfying the laws of unitality and functoriality of a module.
The reason one can see this data as given by a unitary lax double functor is the following. First, the data of the functor in itself is equivalent to that of a unitary lax 2-functor into , i.e. a displayed category (see there for an explanation). This means objects of are sent to categories (the fibers of ) while horizontal maps (tight maps) in act ‘profunctorially’ on them. This assignment is unitary and lax.
Then the action gives, for each vertical morphism (loose maps) in , a functor. Explicitly, if is a vertical map in , and , then .
This part of the action is strict, or at least pseudofunctorial.
One of the central exampls of doubly indexed category is given by the ‘self double-indexing’ any finitely complete category induces. Specifically, it’s the action of .
It is defined as follows:
The laxator is defined by sending squares on and agreeing on their boundary to the obvious composite square lying over (thus forgetting the middle boundary). One can see this assignment is unitary, as is exactly (the identity profunctor on ).
David Jaz Myers, Double Categories of Open Dynamical Systems, EPTCS 333 (2021) 154-167 [arXiv:2005.05956, doi:10.4204/EPTCS.333.11]
David Jaz Myers, Categorical systems theory, book project [github, pdf]
Exposition:
David Jaz Myers, Categorical systems theory, Topos Institute Blog (Nov 2021)
David Jaz Myers, Double Categories of Dynamical Systems (2020) [pdf]
Actions of double categories are central in some approaches to dependent optics?:
Created on December 12, 2022 at 21:40:03. See the history of this page for a list of all contributions to it.