Involution (mathematics)

In mathematics, an involution, involutory function, or self-inverse function^[1] is a function f that is its own inverse,

f(f(x)) = x

for all x in the domain of f.^[2] Equivalently, applying f twice produces the original value.

General properties

Any involution is a bijection.

The

.

The composition g ∘ f of two involutions f and g is an involution if and only if they commute: g ∘ f = f ∘ g.^[3]

Involutions on finite sets

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:

${\displaystyle a_{0}=a_{1}=1}$ and ${\displaystyle a_{n}=a_{n-1}+(n-1)a_{n-2}}$ for ${\displaystyle n>1.}$

The first few terms of this sequence are

The number a_n can also be expressed by non-recursive formulas, such as the sum

$${\displaystyle a_{n}=\sum _{m=0}^{\lfloor {\frac {n}{2}}\rfloor }{\frac {n!}{2^{m}m!(n-2m)!}}.}$$

The number of fixed points of an involution on a finite set and its

Involution throughout the fields of mathematics

Real-valued functions

The graph of an involution (on the real numbers) is symmetric across the line y = x. This is due to the fact that the inverse of any general function will be its reflection over the line y = x. This can be seen by "swapping" x with y. If, in particular, the function is an involution, then its graph is its own reflection. Some basic examples of involutions include the functions

the composition ${\displaystyle f_{4}(x):=(f_{1}\circ f_{2})(x)=(f_{2}\circ f_{1})(x)=-{\frac {1}{x}},}$ and more generally the function is an involution for constants b and c that satisfy bc ≠ −1.

Other nonlinear examples include (note the possibility of interpreting these as compositions):

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

; 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

^: 24

• Any projectivity that interchanges two points is an involution.
• The three pairs of opposite sides of a
  Desargues's Involution Theorem.^[7] Its 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.^[8]
• 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". In the context of projectivities, fixed points are called double points.^[6]: 53

Another type of involution occurring in projective geometry is a polarity that is a correlation of period 2.^[9]

Linear algebra

In linear algebra, an involution is a linear operator T on a vector space, such that T² = 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 that sends e₁ to e₂, and sends e₂ to e₁, and that 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

is also an involution.

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 f² is the identity homomorphism on M.

in a one-to-one manner.

In

with involutions.

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

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:

• complex conjugation on the complex plane, and its equivalent in the split-complex numbers
• taking the transpose in a matrix ring.

Group theory

In group theory, an element of a group is an involution if it has order 2; that is, an involution is an element a such that a ≠ e and a² = e, where e is the identity element.^[10] Originally, this definition agreed with the first definition above, since members of groups were always bijections from a set into itself; that is, 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

.

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

if there is an involution t with x^t = x⁻¹ (where x^t = x⁻¹ = t⁻¹ ⋅ x ⋅ t).

Coxeter groups are groups generated by a set S of involutions subject only to relations involving powers of pairs of elements of S. 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 that 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

involution, it is preserved under conversion.

Computer science

The

bitwise operation is also an involution, and is a special case of the XOR operation where one parameter has all bits set to 1.

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

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

Practically all mechanical cipher machines implement a

, an involution on each typed-in letter. Instead of designing two kinds of machines, one for encrypting and one for decrypting, all the machines can be identical and can be set up (keyed) the same way.^[11]

See also

• Atbash
• Automorphism
• Idempotence
• ROT13

References

Further reading

• Ell, Todd A.; Sangwine, Stephen J. (2007). "Quaternion involutions and anti-involutions". Computers & Mathematics with Applications. 53 (1): 137–143.
  S2CID 45639619
  .
• Knus, Max-Albert;
  Zbl 0955.16001
• "Involution", Encyclopedia of Mathematics, EMS Press, 2001 [1994]

External links

• Media related to Involution at Wikimedia Commons