Yesterday I wrote down a definition of an action of a group on a category that was slightly incorrect because I neglected to write down any conditions involving units. With the definition I gave it is possible for to fail to be an automorphism (it might instead be a nontrivial idempotent endofunctor ).

The condition we need on units is first that we should have a unit isomorphism

and second that this unit isomorphism should be compatible with the isomorphisms in the sense that the composites

and

should both be the identity. If we use the unit isomorphism to replace with (which changes the ), this is just the condition that and should both be the identity.

Similarly, in our definition of an equivalence between two group actions , needs to respect these unit isomorphisms in the sense that

Again, if we use the unit isomorphisms to replace with on the nose, this is just the condition that should be the identity.

Fortunately, in the special case we considered in the previous post, where vanishes (and perhaps in general), this produces the same classification of group actions as before, so nothing has gone too badly wrong. Details below the fold.

**Concrete details**

Recall that in the special case of group actions where each is the identity , the only information left is in the natural isomorphisms . Yesterday we only imposed the usual 2-cocycle condition on these, and saw that equivalences of group actions corresponded to 2-coboundaries. Today we want the additional unit conditions above. This produces a variant of 2-cocycles and 2-coboundaries which we’ll call **unital**. Explicitly, for 2-cocycles this means , and for 2-coboundaries it means we look at 2-coboundaries of the form where .

We want to show that these give rise to the same cohomology group , and we’ll do this very explicitly, by showing that any 2-cocycle is cohomologous to a unital 2-cocycle, and that two unital 2-cocycles are 2-cohomologous iff they are unitally 2-cohomologous. First, using the 2-cocycle condition

and substituting first and then , we get

and

from which it follows that and are both constant, and hence must both be equal to . Now we just need to modify this 2-cocycle by the 2-coboundary where and for ; we get a new 2-cocycle satisfying

and we see that as desired. Hence every 2-cocycle is cohomologous to a unital 2-cocycle.

Next, suppose are two unital 2-cocycles which are cohomologous via a 2-coboundary , so that

.

Then setting gives

and hence must be a unital 2-coboundary.

on January 29, 2020 at 8:02 am |Johannes HahnSo with that in mind, a group action on C is the same as a monodial functor $G \to EndoFun(C)$, right? (Where $G$ is the monodial category with $G$ as objects, only identities as morphisms and multiplication as tensor product)

on March 2, 2020 at 12:17 pm |Qiaochu YuanYes, that’s right, sorry for taking so long to reply!

on November 16, 2015 at 7:55 pm |Stating Galois descent | Annoying Precision[…] 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 […]

on November 11, 2015 at 8:17 pm |Fixed points of group actions on categories | Annoying Precision[…] « Units […]