# Dagger category

In mathematics, a dagger category (also called involutive category or category with involution [1][2]) is a category equipped with a certain structure called dagger or involution. The name dagger category was coined by Selinger.[3]

## Formal definition

A dagger category is a category $\mathcal{C}$ equipped with an involutive functor $\dagger\colon \mathcal{C}^{op}\rightarrow\mathcal{C}$ that is the identity on objects, where $\mathcal{C}^{op}$ is the opposite category.

In detail, this means that it associates to every morphism $f\colon A\to B$ in $\mathcal{C}$ its adjoint $f^\dagger\colon B\to A$ such that for all $f\colon A\to B$ and $g\colon B\to C$,

• $\mathrm{id}_A=\mathrm{id}_A^\dagger\colon A\rightarrow A$
• $(g\circ f)^\dagger=f^\dagger\circ g^\dagger\colon C\rightarrow A$
• $f^{\dagger\dagger}=f\colon A\rightarrow B\,$

Note that in the previous definition, the term "adjoint" is used in a way analogous to (and inspired by) the linear-algebraic sense, not in the category-theoretic sense.

Some sources [4] define a category with involution to be a dagger category with the additional property that its set of morphisms is partially ordered and that the order of morphisms is compatible with the composition of morphisms, that is a<b implies $a\circ c for morphisms a, b, c whenever their sources and targets are compatible.

## Examples

• The category Rel of sets and relations possesses a dagger structure i.e. for a given relation $R:X\rightarrow Y$ in Rel, the relation $R^\dagger:Y\rightarrow X$ is the relational converse of $R$. In this example, a self-adjoint morphism is a symmetric relation.
• A groupoid (and as trivial corollary a group) also has a dagger structure with the adjoint of a morphism being its inverse. In this case, all morphisms are unitary.

## Remarkable morphisms

In a dagger category $\mathcal{C}$, a morphism $f$ is called

• unitary if $f^\dagger=f^{-1}$;
• self-adjoint if $f=f^\dagger$ (this is only possible for an endomorphism $f\colon A \to A$).

The terms unitary and self-adjoint in the previous definition are taken from the category of Hilbert spaces where the morphisms satisfying those properties are then unitary and self-adjoint in the usual sense.

## References

1. ^ M. Burgin, Categories with involution and correspondences in γ-categories, IX All-Union Algebraic Colloquium, Gomel (1968), pp.34–35; M. Burgin, Categories with involution and relations in γ-categories, Transactions of the Moscow Mathematical Society, 1970, v. 22, pp. 161–228
2. ^ J. Lambek, Diagram chasing in ordered categories with involution, Journal of Pure and Applied Algebra 143 (1999), No.1–3, 293–307
3. ^ P. Selinger, Dagger compact closed categories and completely positive maps, Proceedings of the 3rd International Workshop on Quantum Programming Languages, Chicago, June 30–July 1, 2005.
4. ^ Tsalenko, M.Sh. (2001), "Category with involution", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4