Contents

### Context

#### Higher algebra

higher algebra

universal algebra

## Theorems

#### Monoidal categories

monoidal categories

## In higher category theory

#### 2-Category theory

2-category theory

# Contents

## Idea

The idea of a commutative monad could be motivated as either

It turns out that all these concepts coincide (see below, as well as the articles on commutative algebraic theories and monoidal monads).

## Definition

Let $(T,\mu,\eta)$ be a monad on a symmetric monoidal category $C$, equipped with a strength $\sigma$ (and with the costrength $\tau$ induced from $\sigma$ and the braiding).

We say that $T$ (or $\sigma$) is commutative if the following diagram commutes for all objects $X$ and $Y$ of $C$.

A commutative monad is equivalently a monoidal monad, namely a monad in the bicategory of monoidal categories, lax monoidal functors and monoidal natural transformations.

For the details see monoidal monad.

## On closed and monoidal closed categories

One can define strength and costrength on a closed category, in a way that’s analogous to the definition in a monoidal category, and on monoidal closed categories the two definitions coincide (see strong monad - on closed and monoidal closed categories). The same is true for the notion of commutativity: one can define commutative monads on closed categories, and on closed monoidal categories the definition coincides with the definition given in this article.

More in detail, let $C$ be a closed category, and denote its internal homs by $[X,Y]$. Let $t':[X,Y]\to [T X, T Y]$ be a strength for $T$ (as defined here), and let $s':T[X,Y]\to [X, T Y]$ be a costrength (as defined here). We say that $t'$ and $s'$ commute, or that $T$ is commutative, if the following diagram commutes.

It can be shown by currying that on a monoidal closed category, this condition is equivalent to the “monoidal” definition of commutativity. A reference for this is Kock ‘71, Section 2.

## Tensor product of algebras and multimorphisms

In many situations, algebras over a commutative monad are canonically equipped with a tensor product analogous to the tensor product of modules over a ring. In particular, commutative monads come equipped with a notion of multimorphisms of algebras, analogous to bilinear and multilinear maps.

On a closed category, analogously, the category of algebras inherits an internal hom, just like the one of modules. If the category is monoidal closed, the tensor product and the internal hom are adjoint to each other, making the category of algebra itself monoidal closed. This, once again, generalizes the hom-tensor adjunction of modules and vector spaces.

For the details, see tensor product of algebras over a commutative monad, and the section on the universal property for multimorphisms, as well as internal hom of algebras over a commutative monad.

## Examples

In many of the examples, one can see how the property of commutativity (as opposed to just the structure of a strength) has really the meaning of the operations being commutative.

• The free commutative monoid monad on Set is commutative, the free monoid monad is not.
• If $M$ is a monoid, the action monad $X\mapsto M\times X$ on Set is commutative if and only if $M$ is commutative as a monoid.
• More generally, in an arbitrary symmetric monoidal category, the action monad induced by tensoring with an internal monoid is commutative if and only the monoid object is commutative. (See for example Brandenburg, Example 6.3.12.)

• Most probability monads are commutative, with monoidal structure given by forming the product probability?.
• The power set monad is commutative, with monoidal structure given by forming the product of subsets.

For a concrete example, consider the free commutative monoid monad $T$, and given a set $X$, write the elements of $TX$ as formal sums, such as $x_1+x_2$.
The commutativity condition says that these two compositions have the same result,

and

Note that these two expressions are not the same on the nose, they are the same because addition is commutative.

• Anders Kock, Monads on symmetric monoidal closed categories, Arch. Math. 21 (1970), 1–10.

• Anders Kock, Strong functors and monoidal monads, Arhus Universitet, Various Publications Series No. 11 (1970). PDF.

• Anders Kock, Closed categories generated by commutative monads (pdf)

• H. Lindner, Commutative monads in Deuxiéme colloque sur l’algébre des catégories. Amiens-1975. Résumés des conférences, pages 283-288. Cahiers de topologie et géométrie différentielle catégoriques, tome 16, nr. 3, 1975.

• William Keigher, Symmetric monoidal closed categories generated by commutative adjoint monads, Cahiers de Topologie et Géométrie Différentielle Catégoriques, 19 no. 3 (1978), p. 269-293 (NUMDAM, pdf)

• Gavin J. Seal, Tensors, monads and actions (arXiv:1205.0101)

• Martin Brandenburg, Tensor categorical foundations of algebraic geometry (arXiv:1410.1716)