# Multicategory

In mathematics (especially category theory), a **multicategory** is a generalization of the concept of category that allows morphisms of multiple arity. If morphisms in a category are viewed as analogous to functions, then morphisms in a multicategory are analogous to functions of several variables. Multicategories, are also sometimes called operads, or colored operads.

## Definition

A (non-symmetric) multicategory consists of

- a collection (often a proper class) of
*objects*; - for every finite sequence of objects (for von Neumann ordinal ) and object
*Y*, a set of*morphisms*from to*Y*; and - for every object
*X*, a special identity morphism (with*n*= 1) from*X*to*X*.

Additionally, there are composition operations: Given a sequence of sequences of objects, a sequence of objects, and an object *Z*: if

- for each ,
*f*_{j}is a morphism from to*Y*_{j}; and *g*is a morphism from to*Z*:

then there is a composite morphism from to *Z*. This must satisfy certain axioms:

- If
*m*= 1,*Z*=*Y*_{0}, and*g*is the identity morphism for*Y*_{0}, then*g*(*f*_{0}) =*f*_{0}; - if for each ,
*n*_{i}= 1, , and*f*_{i}is the identity morphism for*Y*_{i}, then ; and - an associativity condition: if for each and , is a morphism from to , then are identical morphisms from to
*Z*.

## Comcategories

A *comcategory* (co-multi-category) is a totally ordered set O of objects, a set A of *multiarrows* with two functions

where O^{%} is the set of all finite ordered sequences of elements of O. The dual image of a multiarrow f may be summarized

A comcategory C also has a *multiproduct* with the usual character of a composition operation. C is said to be associative if there holds a *multiproduct axiom* in relation to this operator.

Any multicategory, symmetric *or* non-symmetric, together with a total-ordering of the object set, can be made into an equivalent comcategory.

A *multiorder* is a comcategory satisfying the following conditions.

- There is at most one multiarrow with given head and ground.
- Each object x has a unit multiarrow.
- A multiarrow is a unit if its ground has one entry.

Multiorders are a generalization of partial orders (posets), and were first introduced (in passing) by Tom Leinster.[1]

## Examples

There is a multicategory whose objects are (small) sets, where a morphism from the sets *X*_{1}, *X*_{2}, ..., and *X*_{n} to the set *Y* is an *n*-ary function,
that is a function from the Cartesian product *X*_{1} × *X*_{2} × ... × *X*_{n} to *Y*.

There is a multicategory whose objects are vector spaces (over the rational numbers, say), where a morphism from the vector spaces *X*_{1}, *X*_{2}, ..., and *X*_{n} to the vector space *Y* is a multilinear operator, that is a linear transformation from the tensor product *X*_{1} ⊗ *X*_{2} ⊗ ... ⊗ *X*_{n} to *Y*.

More generally, given any monoidal category **C**, there is a multicategory whose objects are objects of **C**, where a morphism from the **C**-objects *X*_{1}, *X*_{2}, ..., and *X*_{n} to the **C**-object *Y* is a **C**-morphism from the monoidal product of *X*_{1}, *X*_{2}, ..., and *X*_{n} to *Y*.

An operad is a multicategory with one unique object; except in degenerate cases, such a multicategory does not come from a monoidal category.

Examples of multiorders include *pointed multisets* (sequence A262671 in the OEIS), *integer partitions* (sequence A063834 in the OEIS), and *combinatory separations* (sequence A269134 in the OEIS). The triangles (or compositions) of any multiorder are morphisms of a (not necessarily associative) category of *contractions* and a comcategory of *decompositions*. The contraction category for the multiorder of *multimin partitions* (sequence A255397 in the OEIS) is the simplest known category of multisets.[2]

## Applications

Multicategories are often incorrectly considered to belong to higher category theory, as their original application was the observation that the operators and identities satisfied by higher categories are the objects and multiarrows of a multicategory. The study of n-categories was in turn motivated by applications in algebraic topology and attempts to describe the homotopy theory of higher dimensional manifolds. However it has mostly grown out of this motivation and is now also considered to be part of pure mathematics.

The correspondence between contractions and decompositions of triangles in a multiorder allows one to construct an associative algebra called its *incidence algebra*. Any element that is nonzero on all unit arrows has a compositional inverse, and the *Möbius function* of a multiorder is defined as the compositional inverse of the zeta function (constant-one) in its incidence algebra.

## History

Multicategories were first introduced under that name by Jim Lambek in "Deductive systems and categories II" (1969)[3] He mentions (p. 108) that he was "told that multicategories have also been studied by [Jean] Benabou and [Pierre] Cartier", and indeed Leinster opines that "the idea might have occurred to anyone who knew what both a category and a multilinear map were".[1]^{:63}

## References

- Tom Leinster (2004).
*Higher Operads, Higher Categories*. Cambridge University Press. arXiv:math/0305049. Bibcode:2004hohc.book.....L., Example 2.1.7, page 37 - Wiseman, Gus. "Comcategories and Multiorders". Google Docs. Retrieved 9 May 2016.
- .Lambek, Joachim (1969). "Deductive systems and categories II. Standard constructions and closed categories".
*Lecture Notes in Mathematics*. Berlin, Heidelberg: Springer Berlin Heidelberg. doi:10.1007/bfb0079385. ISBN 978-3-540-04605-9. ISSN 0075-8434.

- Garner, Richard (2008). "Polycategories via pseudo-distributive laws".
*Advances in Mathematics*.**218**(3): 781–827. arXiv:math/0606735. doi:10.1016/j.aim.2008.02.001.