Contents

category theory

# Contents

## Definition

$FinRel$ is the category of finite sets and all relations between them: the full subcategory of Rel on finite sets. Like Rel, FinRel can also be seen as a 2-poset, and a cartesian bicategory.

## Properties

### As a category of matrices

$FinRel$ is equivalent to $Mat(Bool)$, the category whose objects are natural numbers and where a morphism $f \colon m \to n$ is an $n \times m$ matrix with entries in the semiring of booleans, $Bool = \{F,T\}$, where addition is the logical operation “or” and multiplication is “and”.

For any semiring $R$, the category $Mat(R)$ has biproducts, corresponding to addition of natural numbers, and the initial object $0$ is also terminal (hence is a zero object). For any commutative semiring $R$, $Mat(R)$ also has another symmetric monoidal structure: a tensor product with $m \otimes n = m n$, which distributes over biproducts. Thus $FinRel$ has all these properties.

### As a PROP

We can define bimonoids in any symmetric monoidal category: roughly, a bimonoid is a monoid and comonoid in a compatible way. A bimonoid is said to be special if comultiplication followed by multiplication is the identity, and bicommutative if it is both a commutative monoid and a cocommutative comonoid.

In a category with biproducts and a zero object, every object $c$ is a bicommutative bimonoid where the multiplication is the fold map $\nabla \colon c \oplus c \to c$ and the comultiplication is the diagonal $\Delta \colon c \to c \oplus c$. Thus, every object of $FinRel$ is a bicommutative bimonoid. One can easily check that it is also special.

But something stronger is true. $FinRel$ made symmetric monoidal with biproducts is the free symmetric monoidal category on a special bicommutative bimonoid. That is, given a symmetric monoidal category $C$ and a special bicommutative bimonoid object $c \in C$, there is symmetric monoidal functor $F \colon FinRel \to C$, unique up to monoidal natural isomorphism, mapping $1 \in FinRel$ together with its bimonoid operations to $c$ and its bimonoid operations.

This is conveniently expressed in the language of PROPs: $Mat(Bool)$ is the PROP for special bicommutative bimonoids. This is Theorem 7.2 in Coya-Fong 17, based on techniques from Lack 04.

• Steve Lack, Composing PROPs, Theory and Applications of Categories 13(9):147–163, 2004. (pdf)

• Brandon Coya and Brendan Fong, Corelations are the prop for extraspecial commutative Frobenius monoids, Theory and Applications of Categories, Vol. 32, 2017, No. 11, pp. 380-395. (arxiv)

category: category