# Symmetric monoidal category

In category theory, a branch of mathematics, a symmetric monoidal category is a monoidal category (i.e. a category in which a "tensor product" $\otimes$ is defined) such that the tensor product is symmetric (i.e. $A\otimes B$ is, in a certain strict sense, naturally isomorphic to $B\otimes A$ for all objects $A$ and $B$ of the category). One of the prototypical examples of a symmetric monoidal category is the category of vector spaces over some fixed field k, using the ordinary tensor product of vector spaces.

## Definition

A symmetric monoidal category is a monoidal category (C, ⊗, I) such that, for every pair A, B of objects in C, there is an isomorphism $s_{AB}:A\otimes B\to B\otimes A$ that is natural in both A and B and such that the following diagrams commute:

In the diagrams above, a, l , r are the associativity isomorphism, the left unit isomorphism, and the right unit isomorphism respectively.

## Examples

Some examples and non-examples of symmetric monoidal categories:

• The category of sets. The tensor product is the set theoretic cartesian product, and any singleton can be fixed as the unit object.
• The category of groups. Like before, the tensor product is just the cartesian product of groups, and the trivial group is the unit object.
• More generally, any category with finite products, that is, a cartesian monoidal category, is symmetric monoidal. The tensor product is the direct product of objects, and any terminal object (empty product) is the unit object.
• The category of bimodules over a ring R is monoidal (using the ordinary tensor product of modules), but not necessarily symmetric. If R is commutative, the category of left R-modules is symmetric monoidal. The latter example class includes the category of all vector spaces over a given field.
• Given a field k and a group (or a Lie algebra over k), the category of all k-linear representations of the group (or of the Lie algebra) is a symmetric monoidal category. Here the standard tensor product of representations is used.
• The categories (Ste,$\circledast$ ) and (Ste,$\odot$ ) of stereotype spaces over ${\mathbb {C} }$ are symmetric monoidal, and moreover, (Ste,$\circledast$ ) is a closed symmetric monoidal category with the internal hom-functor $\oslash$ .

## Properties

The classifying space (geometric realization of the nerve) of a symmetric monoidal category is an $E_{\infty }$ space, so its group completion is an infinite loop space.

## Specializations

A dagger symmetric monoidal category is a symmetric monoidal category with a compatible dagger structure.

A cosmos is a complete cocomplete closed symmetric monoidal category.

## Generalizations

In a symmetric monoidal category, the natural isomorphisms $s_{AB}:A\otimes B\to B\otimes A$ are their own inverses in the sense that $s_{BA}\circ s_{AB}=1_{A\otimes B}$ . If we abandon this requirement (but still require that $A\otimes B$ be naturally isomorphic to $B\otimes A$ ), we obtain the more general notion of a braided monoidal category.