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A modular tensor category is defined to be a ribbon fusion category $\mathcal{C}$ in which the only objects which commute with all other objects are multiples of the identity. That is, if we denote by $\beta_{A,B}:A\otimes B\to B\otimes A$ the braiding on $\mathcal{C}$, then $\beta_{B,A}\circ\beta_{A,B}=id_{A\otimes B}$ for all $B\in\mathcal{C}$ if and only if $A=n\cdot 1$ for some natural number $n\in \mathbb{Z}_{\geq 0}$, where $1$ denotes the tensor unit.

Similarly, a factor is defined to be a von Neumann algebra $M$ in whuch the only elements which commute with all other elements are multiples of the identity. That is, if $AB=BA$ for all $B\in M$ then $A=\lambda \cdot 1$ for some $\lambda\in \mathcal{C}$, where $1$ is the algebra unit.

Of course, factors and modular tensor categories not unrelated mathematical objects. There is a deep connection between modular tensor categories and (sub)factors, discussed for instance in Muger's 2002 paper. Seeing as I am a beginner to subfactor theory, I am still a bit shaky on how this interplay works.

Is there a direct way of going from the trivial center condition on modular tensor categories and the trivial center condition on factors? Are these guises of the same phenominon?

This seems especially relevant to me since these trivial center conditions are the most subtle conditions in the theory. For tensor categories, a few explanations of why one requires there to be no transparent objects are found on an earlier MathOverflow question.

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Disclaimer: This is a long-winded attempt to answer the "... Are these guises of the same phenomenon?" part of the question. I do not understand how to translate between the two.


There is a theory of $E_n$ algebras that describes the collection of all algebras that have '$n$ dimensions worth of multiplications'. One way of making this idea precise is to take the category $\text{Disk}^n$, whose objects are disjoint unions of $n$-disks, and whose morphisms are embeddings. The operation of disjoint union gives this category a symmetric monoidal structure. If we allow for homotopies of embeddings, and homotopies of such homotopies, we can think of $\text{Disk}^n$ as a symmetric monoidal $(\infty,1)$-category.

If $\mathcal S$ is another symmetric monoidal $\infty$-category, then an $E_n$ algebra $F$ valued in $\mathcal S$ is just a symmetric monoidal functor $$F:\text{Disk}^n\longrightarrow\mathcal S\;.$$ We often conflate the functor with the object $\mathcal C=F(\mathbf{D}^n)$ in $\mathcal S$. The fact that two disks can be embedded into a single disk gives rise to a multiplication map $\mathcal C\otimes\mathcal C\to \mathcal C$. The fact that two disks can be embedded into a single disk in multiple ways means that there are multiple such multiplications on $\mathcal C$, and the fact that some embeddings are homotopic gives rise to things like braidings, etc.

Unpacking this definition for $n=1$ and $2$, and $\mathcal S=\text{Vec}$ (=vector spaces) and $2\text{Vec}$ (= semisimple linear categories), we find the following table:

$\downarrow n$ $\hspace{2mm}$ $\mathcal S\rightarrow$ $\text{Vec}$ $2\text{Vec}$
1 Algebra Tensor Category
2 Commutative Algebra Braided Tensor Category
3 Commutative Algebra Symmetric Tensor Category

The center of an $E_n$ algebra $\mathcal C$ is an $E_{n+1}$ algebra denoted $\mathcal Z(\mathcal C)$. The definition is a bit technical, so I won't go into it here, but you can find it on page 4 of [Brochier,Jordan,Safronov,Snyder]. The center of an algebra is precisely the classical notion of center. The center of a tensor category is the Drinfel'd center. The center of a braided tensor category is the Müger center.

For $E_1$-algebras, the notions of central simple algebras, Azumaya algebras, and factors are all different incarnations of the condition that $\mathcal Z(\mathcal C)=\mathbf 1$. For $E_2$-algebras, the condition that $\mathcal Z(\mathcal C)=\mathbf 1$ is sometimes referred to as being modular, or factorizable. In general $\mathcal Z(\mathcal C)=\mathbf 1$ is related to higher Morita invertibility.

You may have heard of the fact that all Drinfel'd centers are modular. This fact is a special case of the more general idea that $\mathcal Z^2$ ought to always be trivial. Here I use the word idea instead of the word fact, because I believe that extra hypotheses need to be added to the theory in order to make this work. Either extra assumptions on $\mathcal S$, or perhaps extra structure on the disks, such as framing, oriantability etc. At any rate, for the purposes of TQFTs, a physical argument for why $\mathcal Z^2=\mathbf 1$ is given as Corollary 7 in [Kong,Wen]. There they lay out a vision of $\mathcal Z$ being the coboundary map of a chain complex $$\cdots E_{n-2}\mathop{\longrightarrow}\limits^{\mathcal Z}E_{n-1}\mathop{\longrightarrow}\limits^{\mathcal Z}E_{n}\mathop{\longrightarrow}\limits^{\mathcal Z}E_{n+1}\mathop{\longrightarrow}\limits^{\mathcal Z}\cdots$$

In this perspective, the statement that an algebra is a factor is just the statement that it is a 1-cocycle, and the condition that a braided category is modular is just that it is a 2-cocycle.

As a follow up, I can give you two examples where the resulting cohomology groups are interesting. For semisimple $E_n$ algebras in $\mathcal S=2\text{Vec}$ (over the complex numbers), the homology group $$\frac{\text{ker}\big(E_2\mathop{\longrightarrow}\limits^{\mathcal Z}E_3\big)}{\text{im}\big(E_1\mathop{\longrightarrow}\limits^{\mathcal Z}E_2\big)}$$ is precisely the Witt group of [Davydov,Müger,Nikshych,Ostrik].

For $E_n$ algebras in $\mathcal S=\text{Vec}$, the homology group $$\frac{\text{ker}\big(E_1\mathop{\longrightarrow}\limits^{\mathcal Z}E_2\big)}{\text{im}\big(E_0\mathop{\longrightarrow}\limits^{\mathcal Z}E_1\big)}$$ counts central simple algebras modulo matrix algebras. In other words it is precisely the Brauer group of the base field.

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    $\begingroup$ This is fantastic - thank you for the detailed explanation! Definitely changes the way I view the relationship between factors and modular tensor categories. I guess the subtlety between the trivial center condition on factors and the trivial center condition on modular tensor categories is that not only do the choices of $\mathcal{S}$ increase on the categorical level, but also the $n$ in the $E_n$ changes - two sorts of generalizations going on at once! $\endgroup$
    – Milo Moses
    Commented Sep 15, 2023 at 5:42

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