# Involution (mathematics)

An involution is a function ${\displaystyle f:X\to X}$ that, when applied twice, brings one back to the starting point.

In mathematics, an involution, or an involutory function, is a function f that is its own inverse,

f(f(x)) = x

for all x in the domain of f.[1]

The term anti-involution refers to involutions based on antihomomorphisms (see below the section on Quaternion algebra, groups, semigroups)

f(xy) = f(y) f(x)

such that

xy = f(f(xy)) = f( f(y) f(x) ) = f(f(x)) f(f(y)) = xy.

## General properties

Any involution is a bijection.

The identity map is a trivial example of an involution. Common examples in mathematics of nontrivial involutions include multiplication by −1 in arithmetic, the taking of reciprocals, complementation in set theory and complex conjugation. Other examples include circle inversion, rotation by a half-turn, and reciprocal ciphers such as the ROT13 transformation and the Beaufort polyalphabetic cipher.

The number of involutions, including the identity involution, on a set with n = 0, 1, 2, ... elements is given by a recurrence relation found by Heinrich August Rothe in 1800:

a0 = a1 = 1;
an = an − 1 + (n − 1)an − 2, for n > 1.

The first few terms of this sequence are 1, 1, 2, 4, 10, 26, 76, 232 (sequence A000085 in the OEIS); these numbers are called the telephone numbers, and they also count the number of Young tableaux with a given number of cells.[2] The composition gf of two involutions f and g is an involution if and only if they commute: gf = fg.[3]

Every involution on an odd number of elements has at least one fixed point. More generally, for an involution on a finite set of elements, the number of elements and the number of fixed points have the same parity.[4]

## Involution throughout the fields of mathematics

### Pre-calculus

Basic examples of involutions are the functions:

${\displaystyle f_{1}(x)=-x}$,   or   ${\displaystyle f_{2}(x)={\frac {1}{x}}}$, as well as their composition ${\displaystyle (f_{1}\circ f_{2})(x)=(f_{2}\circ f_{1})(x)=f_{3}(x)=-{\frac {1}{x}}.}$

These are not the only pre-calculus involutions. Another one within the positive reals is:

${\displaystyle f(x)=\ln \left({\frac {e^{x}+1}{e^{x}-1}}\right).}$

The graph of an involution (on the real numbers) is line-symmetric over the line ${\displaystyle y=x}$. This is due to the fact that the inverse of any general function will be its reflection over the 45° line ${\displaystyle y=x}$. This can be seen by "swapping" ${\displaystyle x}$ with ${\displaystyle y}$. If, in particular, the function is an involution, then it will serve as its own reflection.

Other elementary involutions are useful in solving functional equations.

### Euclidean geometry

A simple example of an involution of the three-dimensional Euclidean space is reflection through a plane. Performing a reflection twice brings a point back to its original coordinates.

Another involution is reflection through the origin; not a reflection in the above sense, and so, a distinct example.

These transformations are examples of affine involutions.

### Projective geometry

An involution is a projectivity of period 2, that is, a projectivity that interchanges pairs of points. Coxeter relates three theorems on involutions:

• Any projectivity that interchanges two points is an involution.
• The three pairs of opposite sides of a complete quadrangle meet any line (not through a vertex) in three pairs of an involution (this is Desargues's Involution Theorem,[5] whose origins can be seen in Lemma IV of the lemmas to the Porisms of Euclid in Volume VII of the Collection of Pappus of Alexandria [6]).
• If an involution has one fixed point, it has another, and consists of the correspondence between harmonic conjugates with respect to these two points. In this instance the involution is termed "hyperbolic", while if there are no fixed points it is "elliptic".

Another type of involution occurring in projective geometry is a polarity which is a correlation of period 2. [7]

### Linear algebra

In linear algebra, an involution is a linear operator T on a vector space, such that ${\displaystyle T^{2}=I}$. Except for in characteristic 2, such operators are diagonalizable for a given basis with just 1s and −1s on the diagonal of the corresponding matrix. If the operator is orthogonal (an orthogonal involution), it is orthonormally diagonalizable.

For example, suppose that a basis for a vector space V is chosen, and that e1 and e2 are basis elements. There exists a linear transformation f which sends e1 to e2, and sends e2 to e1, and which is the identity on all other basis vectors. It can be checked that f(f(x)) = x for all x in V. That is, f is an involution of V.

For a specific basis, any linear operator can be represented by a matrix T. Every matrix has a transpose, obtained by swapping rows for columns. This transposition is an involution on the set of matrices.

The definition of involution extends readily to modules. Given a module M over a ring R, an R endomorphism f of M is called an involution if f 2 is the identity homomorphism on M.

Involutions are related to idempotents; if 2 is invertible then they correspond in a one-to-one manner.

### Quaternion algebra, groups, semigroups

In a quaternion algebra, an (anti-)involution is defined by the following axioms: if we consider a transformation ${\displaystyle x\mapsto f(x)}$ then it is an involution if

• ${\displaystyle f(f(x))=x}$ (it is its own inverse)
• ${\displaystyle f(x_{1}+x_{2})=f(x_{1})+f(x_{2})}$ and ${\displaystyle f(\lambda x)=\lambda f(x)}$ (it is linear)
• ${\displaystyle f(x_{1}x_{2})=f(x_{1})f(x_{2})}$

An anti-involution does not obey the last axiom but instead

• ${\displaystyle f(x_{1}x_{2})=f(x_{2})f(x_{1})}$

This former law is sometimes called antidistributive. It also appears in groups as (xy)−1 = y−1x−1. Taken as an axiom, it leads to the notion of semigroup with involution, of which there are natural examples that are not groups, for example square matrix multiplication (i.e. the full linear monoid) with transpose as the involution.

### Ring theory

In ring theory, the word involution is customarily taken to mean an antihomomorphism that is its own inverse function. Examples of involutions in common rings:

### Group theory

In group theory, an element of a group is an involution if it has order 2; i.e. an involution is an element a such that ae and a2 = e, where e is the identity element.[8]

Originally, this definition agreed with the first definition above, since members of groups were always bijections from a set into itself; i.e., group was taken to mean permutation group. By the end of the 19th century, group was defined more broadly, and accordingly so was involution.

A permutation is an involution precisely if it can be written as a product of one or more non-overlapping transpositions.

The involutions of a group have a large impact on the group's structure. The study of involutions was instrumental in the classification of finite simple groups.

An element x of a group G is called strongly real if there is an involution t with xt = x−1 (where xt = t−1xt).

Coxeter groups are groups generated by involutions with the relations determined only by relations given for pairs of the generating involutions. Coxeter groups can be used, among other things, to describe the possible regular polyhedra and their generalizations to higher dimensions.

### Mathematical logic

The operation of complement in Boolean algebras is an involution. Accordingly, negation in classical logic satisfies the law of double negation: ¬¬A is equivalent to A.

Generally in non-classical logics, negation that satisfies the law of double negation is called involutive. In algebraic semantics, such a negation is realized as an involution on the algebra of truth values. Examples of logics which have involutive negation are Kleene and Bochvar three-valued logics, Łukasiewicz many-valued logic, fuzzy logic IMTL, etc. Involutive negation is sometimes added as an additional connective to logics with non-involutive negation; this is usual, for example, in t-norm fuzzy logics.

The involutiveness of negation is an important characterization property for logics and the corresponding varieties of algebras. For instance, involutive negation characterizes Boolean algebras among Heyting algebras. Correspondingly, classical Boolean logic arises by adding the law of double negation to intuitionistic logic. The same relationship holds also between MV-algebras and BL-algebras (and so correspondingly between Łukasiewicz logic and fuzzy logic BL), IMTL and MTL, and other pairs of important varieties of algebras (resp. corresponding logics).

In the study of binary relations, every relation has a converse relation. Since the converse of the converse is the original relation, the conversion operation is an involution on the category of relations. Binary relations are ordered through inclusion. While this ordering is reversed with the complementation involution, it is preserved under conversion.

### Computer science

The XOR bitwise operation with a given value for one parameter is also an involution. XOR masks were once used to draw graphics on images in such a way that drawing them twice on the background reverts the background to its original state.

Another example is a bit mask and shift function operating on color values stored as integers say in the form RGB that swaps R and B, resulting in form BGR. f(f(RGB))=RGB, f(f(BGR))=BGR.

The RC4 cryptographic cipher is involutionary, as encryption and decryption operations use the same function.