Two operations, and , of an algebraic theory are said to commute if for any matrix of elements, with the number of rows given by the arity of and the number of columns by the arity of , one gets the same result whether one
applies to each column of and then to the resulting row, or
applies to each row of and then to the resulting column.
(We formulate this notion in an element-free way below.)
Note that an operation of arity or always commutes with itself; this is not necessarily the case for higher arities. Commuting nullary operations are necessarily equal.
The operations that commute with a given set of operations in an algebraic theory form a subtheory. The centre of an algebraic theory is given by the operations that commute with all the operations of the theory. An algebraic theory is commutative if every pair of its operations commute. Another way of describing the centre is to say that it consists of those operations which are also homomorphisms; an algebraic theory is commutative if all of its operations are homomorphisms.
Here is a more formal definition, expressed in terms of structure on the monad associated with the algebraic theory.
Here denotes the strength of the monad , and its symmetric counterpart.
is the composite
It is worth checking what this description gives more explicitly. Starting with a pair of elements in ,
(and here we should be considering equivalence classes of such formal operations), the map sends this to
where is the evident operation of arity . In more detail, is the end result of the sequence
Similarly, the map sends the pair to
where is the evident operation of arity .
The category of Lawvere theories is endowed with a symmetric monoidal product (called Kronecker product; see Freyd’s article in the references),
whereby -algebras are -algebras internal to -algebras, or equally well -algebras internal to -algebras. A commutative theory is tantamount to a commutative monoid in the symmetric monoidal category .
If and are commutative theories, then their coproduct in the category of commutative theories is .
Commutative theory as monoidal monad
Let be the -monad of a commutative theory. Then the map
as defined above can be shown to be the structure map for a monoidal structure on , i.e., making a lax (symmetric) monoidal functor, and in fact the monad multiplication and unit become monoidal transformations. In other words, we get a monad in the 2-category of symmetric monoidal categories, lax symmetric monoidal functors, and monoidal transformations: a monoidal monad.
In fact, it may be shown that commutative Lawvere theories on are precisely the same things as (finitary) symmetric monoidal monad structures on , as shown by Anders Kock. For more on this, see monoidal monad.
As commutative monoid in a duoidal category
The commutativity of a theory can also be expressed as an abstract property of a monoid in a duoidal category, specialized to the duoidal category of finitary endofunctors.
If are homomorphisms of models (algebras) of a commutative algebraic theory, and is an -ary operation of it, then the function given by sending to is again a homomorphism, which is naturally called . In this way is enriched as a model of the algebraic theory, and we have a closed category of models and homorphisms. Furthermore, this internal has a left adjoint for which the free model on one generator is a unit, so we have a closed monoidal category, in fact a closed symmetric monoidal category.
The Kronecker product of theories was introduced in an article of Freyd:
Peter Freyd, Algebra valued functors in general and tensor products in particular, Colloq. Math. 14 (1966), 89-106.
Recently Nikolai Durov rediscovered that notion for the purposes of geometry (under the name commutative algebraic monad), constructed their spectra (generalizing the spectrum of Grothendieck) and theory of generalized schemes on this basis. There is a generalized version of the Eckmann–Hilton argument concerning commutative finitary monads. Much detail including many examples and further constructions are in his thesis