# Contents

## Idea

An involution is an endomorphism whose square is the identity morphism. Such an endomorphism must be an automorphism; indeed, it is its own inverse.

Where this makes sense, an anti-involution is an antihomomorphism instead of a homomorphism (so an antiendomorphism and necessarily an antiautomorphism).

## Properties

### Commuting involutions

Two involutions $f, g : X \to X$ commute if and only if their composition $f g$ is also an involution, as displayed by the following algebra:

$f g = f (f g f g ) g = (f f) g f (g g) = g f$
$(f g)(f g) = f (g f) g = f (f g) g = (f f)(g g)= 1$

### Fixed point free involutions

In combinatorics, an important class of involutions are the fixed point free ones: an arbitrary involution on a finite set of cardinality $n$ may be specified by the choice of $k$ elements which are fixed together with a fixed point free involution on the remaining $(n-k)$. The number of fixed point free involutions on a set of $2n$ labelled elements is counted by the double factorial $(2n-1)!! = (2n-1)\cdot (2n-3)\cdot\dots\cdot 3\cdot 1 = \frac{(2n)!}{2^n n!}$, while arbitrary involutions on a set of $n$ labelled elements are counted by OEIS sequence A000085, which also counts the number of Young tableaux with $n$ cells.

An involution on a set $X$ is the same thing as an action of $\mathbb{Z}/2\mathbb{Z}$ on $X$.

More generally, let $(C,\otimes,1)$ be a monoidal category with distributive finite coproducts. The object $2 = 1 + 1$ is equipped with an involution

$not : 2 \to 2$

defined as the copairing $not = [inr,inl]$ of the right and left injections. Moreover, 2 can be given the structure of a monoid in $C$, with unit and multiplication

$false : 1 \to 2 \qquad xor : 2 \otimes 2 \to 2$

defined by $false = inl$ and $xor = [id,not]$ (here we make use of the isomorphism $2 \otimes 2 \cong 2 + 2$ to define $xor$ by copairing). The mapping

$X \mapsto 2 \otimes X \cong X + X$

thus extends to a monad on $C$, sending any object $X$ to the free object equipped with an involution over $X$. Explicitly, the unit $\eta_X : X \to 2\otimes X$ and the multiplication $\mu_X : 2\otimes 2\otimes X \to 2\otimes X$ of the monad are defined by tensoring the unit and the multiplication of the monoid with the identity on $X$, while the involution on $2 \otimes X$ is likewise defined by tensoring the involution on 2 with the identity on $X$.

We then have that involutions in $C$ are precisely the algebras of the monad $(2\otimes-,false\otimes-,xor\otimes-)$. In the forward direction, given an involution $f : X \to X$, we define a monad algebra structure $\alpha : 2\otimes X \to X$ on $X$ by $\alpha = [id,f]$ (again using the isomorphism $2\otimes X \cong X+X$). Conversely, given a monad algebra $\alpha : 2\otimes X \to X$, we can define an endomorphism $f : X \to X$ by $f = \alpha \circ inr$. The monad algebra laws imply that

$\alpha \circ inr \circ \alpha \circ inr = \alpha \circ (2\otimes \alpha) \circ (2\otimes inr) \circ inr = \alpha \circ (xor\otimes id) \circ (2\otimes inr) \circ inr$

and since $xor$ is defined such that $(xor\otimes id) \circ (2\otimes inr) \circ inr = id$, we derive that $\alpha \circ inr$ is an involution.

• Philippe Flajolet and Robert Sedgewick, Analytic Combinatorics, CUP, 2009. (author pdf)