# nLab Drinfeld center

Contents

### Context

#### Monoidal categories

monoidal categories

## In higher category theory

#### 2-Category theory

2-category theory

# Contents

## Idea

The notion of center of a monoidal category or Drinfeld center is the categorification of the notion of center of a monoid(associative algebra, group, etc.) from monoids to monoidal categories.

Where the center of a monoid is just a sub-monoid with the property that it commutes with everything else, under categorification this becomes a stuff, structure, property, since we have to specify how the objects in the Drinfeld center commute (braid) with everything else.

## Definition

We first give the general-abstract definition

of Drinfeld centers. Then we spell out what this means in components in

### Abstractly

###### Definition

For $(\mathcal{C}, \otimes)$ a monoidal category, write $\mathbf{B}_\otimes \mathcal{C}$ for its delooping, the pointed 2-category with a single object $*$ such that $Hom_{\mathbf{B}_\otimes \mathcal{C}}(*, *) \simeq \mathcal{C}$.

The Drinfeld center $Z(\mathcal{C}, \otimes)$ of $(\mathcal{C}, \otimes)$ is the monoidal category of endo-pseudonatural transformations of the identity-2-functor on $\mathbf{B}_\otimes \mathcal{C}$:

$Z(\mathcal{C}, \otimes) \coloneqq End_{\mathbf{B}_\otimes \mathcal{C}}(id_{\mathbf{B}_\otimes \mathcal{C}}) \,.$
###### Remark

Unwinding the definitions, we find that an object of $Z(\mathcal{C}, \otimes)$, $\Phi \colon id_{\mathbf{B}_\otimes \mathcal{C}} \to id_{\mathbf{B}_\otimes \mathcal{C}}$, has for components pseudonaturality squares

$\array{ \ast &\stackrel{X \coloneqq \Phi(\ast)}{\to}& \ast \\ {}^{\mathllap{Y}}\downarrow &\swArrow_{\Phi_Y}& \downarrow^{\mathrlap{Y}} \\ \ast &\underset{X = \Phi(\ast)}{\to}& \ast }$

for each $Y \in Obj(\mathcal{C})$. As shown, these consist of a choice of an object $X \in \mathcal{C}$ together with a natural isomorphism

$\Phi_{(-)} \colon X \otimes (-) \to (-) \otimes X$

in $\mathcal{C}$.

The transfor-property of $\Phi$ says that

$\array{ \ast &\stackrel{X \coloneqq \Phi(\ast)}{\to}& \ast \\ {}^{\mathllap{Y}}\downarrow &\swArrow_{\Phi_Y}& \downarrow^{\mathrlap{Y}} \\ \ast &\underset{X = \Phi(\ast)}{\to}& \ast \\ {}^{\mathllap{Z}}\downarrow &\swArrow_{\Phi_Z}& \downarrow^{\mathrlap{Z}} \\ \ast &\underset{X = \Phi(\ast)}{\to}& \ast } \;\;\;\; \simeq \;\;\;\; \array{ \ast &\stackrel{X \coloneqq \Phi(\ast)}{\to}& \ast \\ {}^{\mathllap{Y \otimes Z}}\downarrow &\swArrow_{\Phi_{Y \otimes Z}}& \downarrow^{\mathrlap{Y \otimes Z}} \\ \ast &\underset{X = \Phi(\ast)}{\to}& \ast } \,.$

And so forth. Writing this out in terms of $(\mathcal{C}, \otimes)$ yields the following component characterization of Drinfeld centers, def. .

### In components

###### Definition

Let $(\mathcal{C}, \otimes)$ be a monoidal category. Its Drinfeld center is a monoidal category $Z(\mathcal{C})$ whose

• objects are pairs $(X, \Phi)$ of an object $X \in \mathcal{C}$ and a natural isomorphism (braiding morphism)

$\Phi \colon X \otimes (-) \to (-) \otimes X$

such that for all $Y \in \mathcal{C}$ we have

$\Phi_{Y \otimes Z} = (id \otimes \Phi_Z) \circ (\Phi_Y \otimes id)$
• morphisms are given by

$Hom((X, \Phi), (Y,\Psi)) = \left\{ f \in Hom_{\mathcal{C}}(X,Y) \;|\; (id \otimes f) \circ \Phi_Z = \Psi_Z \circ (f \otimes id), \; \forall Z \in \mathcal{C} \right\} \,.$
• the tensor product is given by

$(X, \Phi) \otimes (Y, \Psi) = (X \otimes Y, (\Phi \otimes id) \circ (id \otimes \Psi)) \,.$

## Properties

### Extra structure on the Drinfeld center

###### Proposition

The Drinfeld center $Z(\mathcal{C})$ is naturally a braided monoidal category.

###### Proposition

If $\mathcal{C}$ is a fusion category then the Drinfeld center $Z(\mathcal{C})$ is also naturally a fusion category.

### Relation to Drinfeld double under Tannaka duality

Under Tannaka duality, forming the Drinfeld center of a category of modules of some Hopf algebra corresponds to forming the category of modules over the corresponding Drinfeld double algebra. See there for more.

Tannaka duality for categories of modules over monoids/associative algebras

monoid/associative algebracategory of modules
$A$$Mod_A$
$R$-algebra$Mod_R$-2-module
sesquialgebra2-ring = monoidal presentable category with colimit-preserving tensor product
bialgebrastrict 2-ring: monoidal category with fiber functor
Hopf algebrarigid monoidal category with fiber functor
hopfish algebra (correct version)rigid monoidal category (without fiber functor)
weak Hopf algebrafusion category with generalized fiber functor
quasitriangular bialgebrabraided monoidal category with fiber functor
triangular bialgebrasymmetric monoidal category with fiber functor
quasitriangular Hopf algebra (quantum group)rigid braided monoidal category with fiber functor
triangular Hopf algebrarigid symmetric monoidal category with fiber functor
supercommutative Hopf algebra (supergroup)rigid symmetric monoidal category with fiber functor and Schur smallness
form Drinfeld doubleform Drinfeld center
trialgebraHopf monoidal category

2-Tannaka duality for module categories over monoidal categories

monoidal category2-category of module categories
$A$$Mod_A$
$R$-2-algebra$Mod_R$-3-module
Hopf monoidal categorymonoidal 2-category (with some duality and strictness structure)

3-Tannaka duality for module 2-categories over monoidal 2-categories

monoidal 2-category3-category of module 2-categories
$A$$Mod_A$
$R$-3-algebra$Mod_R$-4-module

## References

A standard textbook references are

• Shahn Majid, Foundations of quantum group theory, Cambridge Univ. Press
• C. Kassel, Quantum groups

A general discussion of centers of monoid objects in braided monoidal 2-categories (which reduces to the above for the 2-category Cat with its cartesian product) is in

An application to character sheaves is in

In relation to spectra of tensor triangulated categories:

Last revised on October 24, 2020 at 15:37:38. See the history of this page for a list of all contributions to it.