This is a follow up question to the insightful answer by Theo Johnson-Freyd to the question [105221](https://mathoverflow.net/questions/105221/quantum-groups-not-via-presentations).


This answer explained that the quantised enveloping algebra, $U\_q(\mathfrak{g})$, (defined by a presentation due to Jimbo) could not be defined without a choice, say, of a Cartan subalgebra of $\mathfrak{g}$. The answer also made a comment to the effect that the category of (finite dimensional) representations of $U\_q(\mathfrak{g})$ is a deformation quantisation of the category of (finite dimensional) representations of $\mathfrak{g}$. My question is whether this comment can be made precise?


The idea (as I understand it) is that we should put $q=\exp(h)$ and expand in
power series over $h$. Then taking the degree zero part in $h$ gives the representation of $\mathfrak{g}$ and the linear part in $h$ gives some additional structure. If we think of a K-linear category as a K-algebra then
this additional structure would be a Poisson structure. This would also be
a replacement for Drinfeld's Poisson-Lie groups and Lie bialgebras. Then there
would be further structure corresponding to the quasi-triangular structure.


Another line of thought is that Chern-Simons theory is canonically associated
to $\mathfrak{g}$. Does this lead to a canonical construction of the representation theory of $U\_q(\mathfrak{g})$? This would appear to work only
for $q$ a root of unity.


My motivation for this question is that I would like to apply this to Vogel's universal Lie algebra where the Lie algebra is an object in a category and so any choice required for the construction of the quantised enveloping algebra would seem to be unavailable.



Deforming the category of representations of a simple Lie algebra?
I haven't thought about this stuff in a while, but here's what I can remember. (Perhaps I should just read [my notes for a talk on the subject](http://www.math.northwestern.edu/~theojf/CatQuantTalk.pdf) instead.) Before I begin, let me emphasize that nothing I'm about to say is due to me --- much of it dates from my mathematical infancy.


Let me answer your questions a bit out of order.


*Another line of thought is that Chern-Simons theory is canonically associated to $\mathfrak g$. Does this lead to a canonical construction of the representation theory of $U\_q(\mathfrak g)$? This would appear to work only for q a root of unity.*


My memory is that the category $\mathrm{Rep}^{\mathrm{fd}}(U\_q\mathfrak g)$ of finite-dimensional $U\_q\mathfrak g$-modules can be canonically associated to any semisimple Lie algebra $\mathfrak g$ without extra data, in a purely algebraic way. I don't remember how to do this, at least not slickly. When $q = e^\hbar$ is in an infinitesimal neighborhood of $1$, there is a method, probably due to Drinfeld, which works both for $\mathrm{Rep}^{\mathrm{fd}}(U\_q\mathfrak g)$ and for the category of all representations $\mathrm{Rep}(U\_q\mathfrak g)$. More generally, it works for any Lie algebra over a characteristic-$0$ field $\mathbb K$ with a chosen nondegenerate adjoint-invariant symmetric pairing (indeed, only a quadratic Casimir is needed, possibly degenerate). For semisimples, you can take the Killing form.


Here's the coarse outline. Take the category $\mathrm{Rep}\_\hbar(U\mathfrak g) = \mathrm{Rep}(U\mathfrak g) \otimes\_{\mathbb K} \mathbb K[[\hbar ]]$, which is the Kelly–Deligne tensor product of $\mathrm{Rep}(U\mathfrak g)$ and $\mathrm{Rep}(\mathbb K[[\hbar ]])$, along with its standard monoidal functor $\otimes : \mathrm{Rep}\_\hbar(U\mathfrak g) \otimes \mathrm{Rep}\_\hbar(U\mathfrak g) \to \mathrm{Rep}\_\hbar(U\mathfrak g)$, **but not its standard monoidal structure** — I want to modify the associator, and also the braiding. The braiding is modified in some standard way, and the associator is modified by some $O(\hbar)$ term depending on a choice of Drinfeld associator. These is the main topic of the notes I linked above, so I won't go into it now.


Anyway, this category determines the Hopf algebra $U\_q\mathfrak g$ (again with $q = e^\hbar$) up to isomorphism, but not up to canonical isomorphism. That was what I was trying to explain in the answer you linked to.


You are right that Chern–Simons theory also should give a canonical description of $\mathrm{Rep}(U\_q\mathfrak g)$, at least when $q$ is a root of unity. Actually, you should choose a compact group $G$ integrating $\mathfrak g$. Then it's expected, but in general not yet proved, that the semisimplified version of $\mathrm{Rep}(U\_q\mathfrak g)$ is described by the fusion rules for positive-energy representations of the loop group of $G$ (at a level determined by the order of $q$). This latter category is the "path integral / analytic / Chern–Simons" quantization of $\mathfrak g$.


*The idea (as I understand it) is that we should put $q=\exp(h)$ and expand in power series over $h$. Then taking the degree zero part in h gives the representation of and the linear part in h gives some additional structure. If we think of a K-linear category as a K-algebra then this additional structure would be a Poisson structure. This would also be a replacement for Drinfeld's Poisson-Lie groups and Lie bialgebras. Then there would be further structure corresponding to the quasi-triangular structure.*


You can think of the "infinitesimal braiding" determined by the Casimir as being analogous to a Poisson structure. Alternately, you can think of the Casimir itself as analogous to a Poisson structure. This has been made precise in the notion of "shifted symplectic structure" prevalent in the AKSZ literature (see e.g. work by Roytenberg). The category $\mathrm{Rep}(U\mathfrak g)$ "is" the category of quasicoherent sheaves on a "stack" $B \mathfrak g$, the "classifying space" of the formal group determined by $\mathfrak g$. I put a bunch of words in quotes because you should think of formal groups and their classifying spaces as infinitesimal objects (in particular, $B \mathfrak g$ is not a stack in the usual algebrogeometric meaning). The Killing form defines a "shifted symplectic structure" on $B\mathfrak g$, meaning a nondegenerate closed 2-form of some cohomological degree (remember that the tangent bundle to a stack is a bundle of chain complexes, not of vector spaces).


That said, there are some open questions relating general shifted-Poisson structures to structures on the category of quasicoherent sheaves.


*The answer also made a comment to the effect that the category of (finite dimensional) representations of $U\_q(\mathfrak g)$ is a deformation quantisation of the category of (finite dimensional) representations of $\mathfrak g$. My question is whether this comment can be made precise?*


It is probably worth emphasizing, earlier than I am doing it, that for generic $q$ (or $q = e^\hbar$), $\mathrm{Rep}(U\_q\mathfrak g)$ and $\mathrm{Rep}(U\mathfrak g)$ are equivalent *as categories* — the only thing deforming is their braided monoidal structures. Actually, I should be a bit careful: there can be multiply non-isomorphic equivalences, and depending on the context either one has been chosen or it hasn't. Compare the case of deformations of Poisson to associative algebras: we usually assume that the Poisson algebra $A\_0$ and the deformation quantization $A\_\hbar$ are equal *as vector spaces* (perhaps up to tensoring with $\mathbb K[[\hbar]]$), and that that equality is part of the data. Changing that equality by something $O(\hbar)$ is a "gauge transformation" in Kontsevich's sense.


On the other hand, extracting the Poisson structure doesn't require knowing the equality $A\_\hbar = A\_0 \otimes\_{\mathbb K} \mathbb K[[\hbar]]$, just that $A\_\hbar$ is a flat deformation (i.e. $A\_\hbar$ is a sheaf of algebras over the formal disk $\mathrm{Spec}(\mathbb K[[\hbar]])$, and I want to know that as a sheaf of vector spaces it is a vector bundle). Similarly, you can extract the infinitesimal braiding just from $\mathrm{Rep}(U\_q\mathfrak g)$. I state this more precisely in the "Proposition" in section 2 of my linked lecture notes.


But, in any case, yes: the point is that you can present (in many ways) the category $\mathrm{Rep}(U\_q\mathfrak g)$ as having as its objects and morphisms the objects and morphisms of $\mathrm{Rep}(U\mathfrak g)$ (up to base changing), but with its algebraic structure (composition, associator, braiding, etc.) deformed.


