Wrangling Generators for Subobjects
Posted by Emily Riehl
Guest post by John Wiltshire-Gordon
My new paper arXiv:1508.04107 contains a definition that may be of interest to category theorists. Emily Riehl has graciously offered me this chance to explain.
In algebra, if we have a firm grip on some object , we probably have generators for . Later, if we have some quotient , the same set of generators will work. The trouble comes when we have a subobject , which (given typical bad luck) probably misses every one of our generators. We need theorems to find generators for subobjects.
Category theory offers a clean definition of generation: if is some category of algebraic objects and is a free-forgetful adjunction with , then it makes sense to say that a subset generates if the adjunct arrow is epic.
Certainly -modules fit into this setup nicely, and groups, commutative rings, etc. What about simplicial sets? It makes sense to say that some simplicial set is “generated” by its 1-simplices, for example: this is saying that is 1-skeletal. But simplicial sets come with many sorts of generator…Ah, and they also come with many forgetful functors, given by evaluation at the various objects of .
Let’s assume we’re in a context where there are many forgetful functors, and many corresponding notions of generation. In fact, for concreteness, let’s think about cosimplicial vector spaces over the rational numbers. A cosimplicial vector space is a functor , and so for each we have a functor with and left adjoint . We will say that a vector sits in degree , and generally think of as a vector space graded by the objects of .
Definition A cosimplicial vector space is generated in degree if the component at of the counit is epic. Similarly, is generated in degrees if is epic.
Example Let be the free cosimplicial vector space on a single vector in degree . Certainly is generated in degree . It’s less obvious that admits a unique nontrivial subobject . Let’s try to find generators for . It turns out that , so no generators there. Since , there must be generators somewhere… but where?
Theorem (Wrangling generators for cosimplicial abelian groups): If is a cosimplicial abelian group generated in degrees , then any subobject is generated in degrees .
Ok, so now we know exactly where to look for generators for subobjects: exactly one degree higher than our generators for the ambient object. The generators have been successfully wrangled.
The preorder on degrees of generation
Time to formalize. Let be three forgetful functors, and let be their left adjoints. When the labels appear unattached to or , they represent formal “degrees of generation,” even though need not be a functor category. In this broader setting, we say is generated in (formal) degree if the component of the counit is epic. By the unit-counit identities, if is generated in degree , the whole set serves as a generating set.
Definition Say if for all generated in degree , every subobject generated in degree is also generated in degree .
Practically speaking, if , then generators in degree can always be replaced by generators in degree provided that the ambient object is generated in degree .
Suppose that we have a complete understanding of the preorder , and we’re trying to generate subobjects inside some object generated in degree . Then every time , we may replace generators in degree with their span in degree . In other words, the generators are equivalent to generators . Arguing in this fashion, we may wrangle all generators upward in the preorder . If has a finite system of elements capable of bounding any other element from above, then all generators may be replaced by generators in degrees . This is the ideal wrangling situation, and lets us restrict our search for generators to this finite set of degrees.
In the case of cosimplicial vector spaces, is a maximum for the preorder with . So any subobject of a simplicial vector space generated in degree is generated in degree . (It is also true that, for example, is a maximum for the preorder . In fact, we have . That’s why it’s important that is a preorder, and not a true partial order.)
Connection to the preprint arXiv:1508.04107
In the generality presented above, where a formal degree of generation is a free-forgetful adjunction to , I do not know much about the preorder . The paper linked above is concerned with the case of functor categories of -shaped diagrams of -modules. In this case I can say a lot.
In Definition 1.1, I give a computational description of the preorder . This description makes it clear that if has finite hom-sets, then you could program a computer to tell you whenever .
In Section 2.2, I give many different categories for which explicit upper bounds are known for the preorders . (In the paper, an explicit system of upper bounds for every preorder is called a homological modulus.)
Connection to the field of Representation Stability
If you’re interested in more context for this work, I highly recommend two of Emily Riehl’s posts from February of last year on Representation Stability, a subject begun by Tom Church and Benson Farb. With Jordan Ellenberg, they explained how certain stability patterns can be considered consequences of structure theory for the category of -modules where is the category of finite sets with injections. In the category of -modules, the preorders have no finite system of upper bounds. In contrast, for -modules, every preorder has a maximum! (Here is the usual category of finite sets). So having all finite set maps instead of just the injections gives much better control on generators for subobjects. As an application, Jordan and I use this extra control to obtain new results about configuration spaces of points on a manifold. You can read about it on his blog.
For more on the recent progress of representation stability, you can also check out the bibliography of my paper or take a look at exciting new results by CEF, as well as Rohit Nagpal, Andy Putman, Steven Sam, and Andrew Snowden, and Jenny Wilson.
Re: Wrangling generators for subobjects
Thanks for the post. A comment:
I’m not convinced that’s the right definition. Consider the theory of commutative rings (with 1) and the unique ring homomorphism . This is epic, but is it really appropriate to say that “generates” the ring ?
The usual categorical story about presentation of algebras runs as follows. Take some algebraic theory — groups, for concreteness. We have the free-forgetful adjunction , and the induced monad .
Given a set , the set consists of all words in the variables (or “generators”) . So, an equation (or “relation”) in these variables is an element of . So, a family of equations in a set of variables is a set together with a map .
Equivalently, it’s a parallel pair of maps in . Equivalently (by adjointness), it’s a parallel pair of maps in .
So: a group presentation consists of sets and and a pair of homomorphisms . And the group presented by this presentation is exactly the coequalizer of this pair.
This suggests that (in your notation) should be said to generate if the corresponding map is regular epic (that is, a coequalizer). One could perhaps argue for other variants such as strong or extremal epic, but the example above suggests to me that “epic” is too weak.