5
$\begingroup$

Let $\mathcal{C}$ be a symmetric monoidal category. One can imagine a theorem

Tannakian reconstruction: If $\mathcal{B}$ is a braided monoidal category and $F:\mathcal{B}\to \mathcal{C}$ is a functor of braided monoidal categories (with [???] conditions), then $\mathcal{B}\simeq A\text{-Mod}$ for a quasitriangular bialgebra $A\in \mathcal{C}$ and $F$ is the forgetful functor.

Question 1: Where is a reference for this?

Question 2: Where is a reference for the $(\infty,1)$- and $(\infty,2)$-categorical versions of this (if they exist)?

n.b. I am definitely interested in the case of general $\mathcal{C}$, not just $\text{Vect}$.

$\endgroup$

1 Answer 1

3
+150
$\begingroup$

Beware that the forgetful functor on the category of representations of a quasi-triangular Hopf algebra is typically not a braided functor. Rather, the general pattern is that you can reconstruct a bialgebra from a monoidal functor, and then additional structures/properties of the category you started with induces additional structures/properties on that bialgebra. In fact, I claim that $F$ being monoidal plays essentially no role in the sort of conditions you want: once you've identified $B$ with a category of modules for an algebra in $C$, and $F$ with the forgetful functor, then the rest is automatic. Also, in the infinite dimensional setting reconstruction of categories of comodule, rather than modules, usually work better.

That being said, a convenient formalism for that kind of question goes under the name "Barr-Beck monadicity theorem". There are a bunch of versions of those, including version for $\infty$-categories, see e.g. https://ncatlab.org/nlab/show/monadicity+theorem. They give you sufficient condition for $B$ to be the category of modules over a monad on $C$ (there are versions for comonad as well of course).

The trick is that $C$ is a right module over itself and that, in many situation and for an appropriate choice of functors, the map $X \mapsto X \otimes -$ gives a monoidal equivalence $$C \simeq End_C(C)$$ Where the RHS is the monoidal category of right $C$-linear functors. This means that there is an equivalence of categories between algebra objects in $C$, and right $C$-linear monads on $C$. This is how those monadicity theorems can be applied to your situation.

Edit It's hard giving an answer or a reference without knowing the kind of categories you're interested in, but to be a bit more precise: first you need $B$ to be a $C$-module, and $F$ to be a functor of $C$-module. Now a version of the monadicity theorem, adapted to your situation, tells you that:

  • $F$ needs to have a left adjoint $G$ as a $C$-linear functor. In practice this is something you can often check by verifying some conditions on $F$ and $C$. In particular the $C$-linear structure is forced, if there's one compatible with the adjunction.
  • $F$ needs to be conservative. If $B$ is abelian, this is implied by $F$ being faithful, and again this is often how it's checked in practice.
  • $B$ needs to have, and $F$ needs to preserves, coequalizers of $F$-split pairs. Often something much stronger is true by assumption (e.g. $B$ is closed under finite, or all colimits, and $F$ preserves those).
  • you need to know that $FG$, à $C$-linear endofunctor of $C$, is of the form $A \otimes -$ for some object $A \in C$. Again there are many settings in which this is automatic.

Then the unit and the counit of the adjunction make $FG(1_C)=A$ an algebra object in $C$, and $B=A-mod_C$.

Then if $F$ is monoidal, it's well-known that $G$ is automatically oplax monoidal, hence $FG(1_C)=A$ is a coalgebra: this gives you the coproduct. You can then build the R-matrix from the braiding of $B$ and the symmetry of $C$, but there are subtleties in properly defining quasi-triangular bialgebra in arbitrary symmetric monoidal categories, see e.g. https://arxiv.org/abs/math/0604180. However, in concrete cases (e.g. $C=R-mod$ for a commutative ring $R$) this should works the way you'd expect.

$\endgroup$
6
  • $\begingroup$ Thanks for this - "It's hard giving an answer or a reference without knowing the kind of categories you're interested in" - I'm mostly interested in dg categories, or the (\infty,2) category of dg categories (or more generally sheaves of QCoh(X)-modules on some prestack X). $\endgroup$
    – Pulcinella
    Commented Aug 1, 2023 at 17:41
  • $\begingroup$ Also, do you have a reference to a statement of Barr-Beck(-Lurie) monadicity that includes the braided monoidal case I am interested in? $\endgroup$
    – Pulcinella
    Commented Aug 1, 2023 at 17:42
  • 1
    $\begingroup$ I'm definitely more familiar with the discrete rather than $\infty$ setting. By what "kind" of categories, I meant e.g. That there are a lot of references on reconstruction of bialgebra assuming some particular kind of simplifying conditions on the categories at hand (e.g. abelian, finite, linear, rigid, cocomplete under some kind of colimits etc..) while Barr-Beck like theorem are often given in much greater generalities. The claim I was making is that the former can often be recovered fairly easily from the latter using this trick but it's not necessarily written that way. $\endgroup$
    – Adrien
    Commented Aug 2, 2023 at 9:18
  • 1
    $\begingroup$ Also, there are definitely reference on the theory of 2-monads on 2-categories but again I don't know much about that ! $\endgroup$
    – Adrien
    Commented Aug 2, 2023 at 9:20
  • 1
    $\begingroup$ The paper on Hopf monad I mentioned above is the only place I can think of now where they consider quasi-triangular Hopf algebras in a braided category (see also section 8.6 of arxiv.org/abs/0812.2443 by the same authors where they explain it's a bit more complicated than you might think, because objects in a category might not "have enough elements"). The paper arxiv.org/abs/hep-th/9412085 might also be relevant here. $\endgroup$
    – Adrien
    Commented Aug 2, 2023 at 9:32

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .