Yesterday we decided that it might be interesting to describe various categories as “fixed points” of Galois actions on various other categories, whatever that means: for example, perhaps real Lie algebras are the “fixed points” of a Galois action on complex Lie algebras. To formalize this we need a notion of group actions on categories and fixed points of such group actions.

So let be a group and be a category. For starters, we should probably ask for a functor for each . Next, we might naively ask for an equality of functors

but this is too strict: functors themselves live in a category (of functors and natural transformations), and so we should instead ask for natural isomorphisms

.

These natural isomorphisms should further satisfy the following compatibility condition: there are two ways to use them to write down an isomorphism , and these should agree. More explicitly, the composite

should be equal to the composite

.

(There’s also some stuff going on with units which I believe we can ignore here. I think we can just require that on the nose and nothing will go too horribly wrong.)

These natural isomorphisms can be regarded as a natural generalization of 2-cocycles, and the condition above as a natural generalization of a cocycle condition. Below the fold we’ll describe this and other aspects of this definition in more detail, and we’ll end with two puzzles about the relationship between this story and group cohomology.

**Why do this?**

There are several ways to justify this definition. The basic justification is that categories themselves naturally live in a 2-category: accordingly, their endomorphisms form monoidal categories, and we’ve written down above is precisely a (strong) monoidal functor from , regarded as a discrete monoidal category, to . Said another way, the automorphisms form not a group but a **2-group**, and what we’ve written above is the definition of a morphism of 2-groups.

If we’d gone with the more naive definition, we would’ve ended up with a notion of groups acting on categories which wouldn’t have transported across equivalence of categories.

**Where do 2-cocycles come into this?**

Suppose that every is the identity functor . (Abstractly, this is a natural thing to do if every automorphism is equivalent to the identity; then we can replace each with the identity while changing the appropriately.) Then each is a natural isomorphism , or equivalently an element of the group of units of the center , and the compatibility condition becomes

.

Hence, in this special case, the precisely describe a 2-cocycle with coefficients in .

**What about cohomology?**

The point of 2-cocycles is that they can be used to describe second group cohomology; where does that come into the picture?

Abstractly, second group cohomology can be described as homotopy classes of maps , where is the classifying space of and is the classifying space of the classifying space of , or equivalently the Eilenberg-MacLane space . These are the same as homotopy classes of maps of 2-groups (by the homotopy hypothesis), and if the 2-group is connected in the sense that (which, as above, is precisely the condition that every automorphism is equivalent to the identity), then

as 2-groups, and hence in this special case actions of on are classified by group cohomology with coefficient in the group of units of the center of . In general, maps of 2-groups can be thought of as “nonabelian group cohomology,” with coefficients in a 2-group.

Concretely, let’s write down what it ought to mean for two group actions to be equivalent. For starters, we should probably ask for a family of natural isomorphisms

and next we should ask for some compatibility between these isomorphisms and the “2-cocycles” . The natural compatibility to ask for is that the composites

and

are equal. In the special case where all of the are equal to the identity , the isomorphisms again take values in , and the compatibility above becomes

which, with a little rearranging (using the fact that is abelian), becomes

.

This means precisely that the 2-cocycles and differ by a 2-coboundary in the usual sense.

**Okay, but do you have any interesting examples?**

Let be the category of vector spaces over a field . Every automorphism of as a -linear category turns out to be equivalent to the identity. (Without the -linearity condition, any automorphism of induces an automorphism of .) The center is itself, so its group of units is . Hence the set of isomorphism classes of actions of a group on can be identified with the cohomology group

.

**Puzzle 1:** This cohomology group also appears when classifying projective representations of over . Is this a coincidence?

**Puzzle 2:** In terms of the description above as the set of isomorphism classes of actions, where does the group structure on this set come from, and why is it abelian?

on May 31, 2016 at 11:03 am |Higher linear algebra | Annoying Precision[…] where groups (or higher versions of groups, such as 2-groups) act on higher vector spaces. Previously we saw such actions occur naturally in Galois descent: namely, if is a Galois extension with […]

on November 16, 2015 at 7:55 pm |Stating Galois descent | Annoying Precision[…] of functors (where again we’re taking compositions in diagrammatic order) satisfying the usual cocycle condition that the two natural isomorphisms we can write down from this data agree. We’ll also want unit isomorphisms satisfying the same compatibility as before. This is just spelling out the definition of a 2-functor from the category of separable extensions of to the 2-category , and in particular each naturally acquires an action of (where we mean automorphisms of extensions of , hence if is Galois this is the Galois group) in precisely the sense we described earlier. […]

on November 13, 2015 at 6:10 pm |Projective representations give categorical representations | Annoying Precision[…] us to match up the 2-cocycles that are about to appear with the 2-cocycles that appeared when we classified -linear actions of on . Apart from this observation we will no longer need to explicitly talk about the Morita […]

on November 12, 2015 at 7:29 pm |Projective representations | Annoying Precision[…] Three days ago we stated the following puzzle: we can compute that isomorphism classes of -linear actions of a group on the category of vector spaces over a field correspond to elements of the cohomology group […]

on November 11, 2015 at 8:17 pm |Fixed points of group actions on categories | Annoying Precision[…] Previously we described what it means for a group to act on a category (although we needed to slightly correct our initial definition). Today, as the next step in our attempt to understand Galois descent, we’ll describe what the fixed points of such a group action are. […]

on November 10, 2015 at 8:25 pm |Units | Annoying Precision[…] « Group actions on categories […]

on November 10, 2015 at 4:44 am |sureJust some little question about your definition of group acting on a category. The (should I say one?) “good” definition of a group acting on something is given by models of the sketch of a group action into the category you consider (product preserving functors from the sketch of group to the category you’re interested in). You can always take a model of your sketch of a group action in Cat. If you do so, you would have a group-object in Cat acting (in the usual sense) on a category. Why isn’t such definition good for your purposes?

on November 10, 2015 at 11:23 am |Qiaochu YuanBecause Cat isn’t a category, it’s a 2-category, and the correct definition takes this extra structure into account. The naive definition doesn’t, for example, transport across an equivalence of categories; that is, if you have a group acting on a category in the naive sense, and you also have an equivalence of categories , then you do not get a group action in the naive sense on .

on November 10, 2015 at 1:04 pm |sureRegardless of the strictness concerns, I’m not even sure that the definition I propose is equivalent to your when the “group object” is supported over a category with 1 object.

on November 10, 2015 at 1:19 pm |Qiaochu YuanThat’s correct, it’s not, and this distinction is important.

on November 10, 2015 at 12:20 am |Zhen LinYou can require if you want, but I think there is still a compatibility condition: we should require that the composite be equal to the image of the left unitor , and similarly for the right unitor. (Here, is a pseudofunctor between bicategories.) In your case, this amounts to the condition that be the identity when either or is the identity.

Curiously, there is Simpson’s conjecture, which says something to the effect that you can strictify everything except units.

on November 10, 2015 at 11:10 am |Qiaochu YuanHmm, yeah, in fact I’ve been careless. I neglected the difference between maps of spaces and maps of based spaces…

on November 10, 2015 at 8:26 pm |Qiaochu YuanHang on, no, basepoints aren’t the problem. I should just write something about units to be safe.