commutative monad



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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).


Let (T,μ,η)(T,\mu,\eta) be a monad on a symmetric monoidal category CC, equipped with a strength σ\sigma (and with the costrength τ\tau induced from σ\sigma and the braiding).

We say that TT (or σ\sigma) is commutative if the following diagram commutes for all objects XX and YY of CC.

Equivalence with monoidal monads

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 CC be a closed category, and denote its internal homs by [X,Y][X,Y]. Let t:[X,Y][TX,TY]t':[X,Y]\to [T X, T Y] be a strength for TT (as defined here), and let s:T[X,Y][X,TY]s':T[X,Y]\to [X, T Y] be a costrength (as defined here). We say that tt' and ss' commute, or that TT 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.


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.

Since commutative monads are the same as monoidal monads, monads encoding structures which admit products are commutative:

  • 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 TT, and given a set XX, write the elements of TXTX as formal sums, such as x 1+x 2x_1+x_2.
The commutativity condition says that these two compositions have the same result,


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

(See also the explanations at commutative algebraic theory.)

See also


  • 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)

Last revised on February 2, 2020 at 14:34:49. See the history of this page for a list of all contributions to it.