# Definition of a profinite category

When studying objects like profinite groups, profinite spaces and profinite rings, I have noticed that some properties just remain the same. For example they will always be inductive limits of some discrete finite spaces, or equivalently totally disconnected compact Hausdorff spaces.

I was therefore wondering, if there is some kind of a categorical generalization of these notions. Something I would call a profinite category.

I was thinking defining it either in terms of a subcategory of profinite spaces, where the objects would be profinite spaces and morphisms some subsets of continuous maps between two profinite spaces. (For profinite groups we'd require them to be morphisms of groups, for rings morphisms of rings and for modules, morphisms of modules etc...)

Other possible definition, I had in mind would be taking a subcategory of the category of finite sets with some properties that would make it possible to do projective limits and then define the objects in the profinite category as these projective limits.

Are there any references for these kinds of notions?

• One thing which makes it a bit awkward is that finite categories are not closed under certain operations -- for instance a category can be finitely-generated without being finite. Of course, this awkwardness is already present for groups, and that doesn't stop the development of a fruitful theory of profinite groups, so it shouldn't be a showstopper. – Tim Campion Mar 8 at 16:29
• Projective limits rather than inductive limits. Profinite $G$-spaces for a fixed group $G$ are also known as odometers over $G$. For instance, for $G=\mathbf{Z}$, the dynamical system $x\mapsto x+1$ on $n$-adics $\mathbf{Z}_n$. – YCor Mar 8 at 17:11

## 2 Answers

There is a notion of pro-object in a general category $$\mathbf{C}$$, which generalises the usual profinite objects - profinite spaces are the case $$\mathbf{C} = \mathbf{FinSet}$$, profinite groups are $$\mathbf{C} = \mathbf{FinGroup}$$, etc. See this nLab article for details. Perhaps this is what you are looking for.

There are two natural definitions of a profinite category. You can look at inverse limits of finite categories or you can look at topological categories whose underlying spaces are profinite (call these Stone categories). Any profinite category is Stone and any Stone category with finitely many objects is profinite. But there are Stone categories that are not profinite. Many years ago I asked George Bergman for an example, and in typical Bergman fashion he immediately provided me one. It's been many years, so I might be slightly off, but there is a well known example of a Stone lattice that is not a profinite lattice. Any poset can be viewed as a category and I believe just used that poset, viewed as a category, as his example or something very close to that.

Stone categories come up very frequently in monoid theory via taking the free Stone category on a profinite graph. This comes up when trying to understand semidirect product decompositions of monoids.

It is typical given a homomorphism of profinite monoids $$f\colon M\to N$$ to build the category of elements of $$N$$ viewed as a right $$M$$-set with $$M$$ acting on $$N$$ via $$f$$. This is a profinite category and so can be generated by a profinite graph. People then like two write it as a quotient of a free Stone category on that profinite graph to get profinite identity theory involved. But because that free Stone category might not be profinite, troubles arise.

There is lots of work on this by Jorge Almeida and his former students. The original goal was to find profinite identities defining semidirect products of pseudovarieties of monoids, but the approach has not been 100% successful and the profinite versus Stone issue is part of the difficulties.

This paper might give some flavor of what people look at. There are older papers referred to within.

• I guess just for clarification, is the issue that the structural morphisms of a category consisting of profinite spaces (the source, target, composition and units) might not arise as "pro-maps"? Most likely the composition, I guess. I vaguely recall a difference between profinite foos, and foos internal to profinite spaces, where "foos" are a type of algebraic structure, from Johnstone's Stone spaces book. – theHigherGeometer Mar 9 at 0:00
• @DavidRoberts the issue is indeed that categories internal to Stone Spaces is larger than the pro-completion of finite categories – Benjamin Steinberg Mar 9 at 1:11