symmetric monoidal (∞,1)-category of spectra
Sometimes in mathematics we want to consider objects equipped with two different types of extra structure which interact in a suitable way. For instance, a ring is a set equipped with both (1) the structure of an (additive) abelian group and (2) the structure of a (multiplicative) monoid, which satisfy the distributive laws and .
Abstractly, there are two monads on the category Set, one (call it ) whose algebras are abelian groups, and one (call it ) whose algebras are monoids, and so we might ask “can we construct, from these two monads, a third monad whose algebras are rings?” Such a monad would assign to each set the free ring on that set, which consists of formal sums of formal products of elements of —in other words, it can be identified with . Thus the question becomes “given two monads and , what further structure is required to make the composite into a monad?”
It is easy to give a unit, as the composite , but to give it a multiplication we need a transformation from to . We naturally want to use the multiplications and , but in order to do this we first need to switch the order of and . However, if we have a transformation , then we can define to be the composite .
Such a transformation, satisfying suitable axioms to make into a monad, is called a distributive law, because of the motivating example relating addition to multiplication in a ring. In that case, is a formal product of formal sums such as , and the distributive law is given by multiplying out such an expression formally, resulting in a formal sum of formal products such as .
Monads in any 2-category make themselves a 2-category in which 1-cells are either lax or colax morphisms of monads; by dualization the same is true for comonads. Monads internal to the 2-category of monads are called distributive laws. In particular, distributive laws themselves make a 2-category. There are other variants like distributive laws between a monad and an endofunctor, “mixed” distributive laws between a monad and a comonad (the variants for algebras and coalgebras called entwining structures), distributive laws between actions of two different monoidal categories on the same category, for PROPs and so on. Having a distributive law from one monad to another enables to define the composite monad . This correspondence extends to a 2-functor . An analogue of this 2-functor in the mixed setup is a homomorphism of bicategories from the bicategory of entwinings to a bicategory of corings.
A distributive law from a monad in to an endofunctor is a 2-cell such that and . The latter identitity is the commutativity of the pentagon
Distributive laws from the monad to the endofunctor are in a canonical bijection with lifts of to an endofunctor in the Eilenberg-Moore category , satisfying . Indeed, the endofunctor is given by .
A distributive law from a monad to a monad in (or of over ) is a distributive law from to the endofunctor , compatible with in the sense that and . Thus all together a distributive law from a monad to a monad is a 2-cell for which 2 triangles and 2 pentagons commute. In the entwining case, Brzeziński and Majid combined the 4 diagrams into one picture which they call the bow-tie diagram.
Similarly, there are definitions of distributive law of a comonad over a comonad, a monad over a comonad (sometimes called a mixed distributive law), and so on.
More generally, factorization systems over a subcategory can be identified with distributive laws in Prof. Ordinary orthogonal factorization systems are a special case. The latter can also be obtained by other weakenings; see for instance this discussion.
H. Appelgate, Michael Barr, J. Beck, Bill Lawvere, Fred Linton, E, Manes, Myles Tierney, F. Ulmer, Seminar on triples and categorical homology theory, ETH 1966/67, edited by B. Eckmann, LNM 80, Springer 1969. (includes article Jon Beck, Distributive laws, pages 119–140). Republished in Reprints in Theory and Applications of Categories, No. 18 (2008) pp. 1-303. (web)
T. Brzeziński, R. Wisbauer, Corings and comodules, London Math. Soc. Lec. Note Series 309, Cambridge 2003.
T. F. Fox, Martin Markl, Distributive laws, bialgebras, and cohomology, Operads: Proceedings of Renaissance Conferences (Hartford, CT/Luminy, 1995), 167–205, Contemp. Math. 202, AMS 1997.
R. Street, The formal theory of monads, J. Pure Appl. Alg. 2, 149–168 (1972)
Z. Škoda, Distributive laws for monoidal categories (arXiv:0406310); Equivariant monads and equivariant lifts versus a 2-category of distributive laws (arXiv:0707.1609); Bicategory of entwinings (arXiv:0805.4611)
R. Wisbauer, Algebras versus coalgebras, Appl. Categ. Structures 16 (2008), no. 1-2, 255–295.