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Reflexive relation

Binary relation that relates every element to itself

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In mathematics, a binary relation $R$ on a set $X$ is reflexive if it relates every element of $X$ to itself.

An example of a reflexive relation is the relation "is equal to" on the set of real numbers, since every real number is equal to itself. A reflexive relation is said to have the reflexive property or is said to possess reflexivity. Along with symmetry and transitivity, reflexivity is one of three properties defining equivalence relations.

01Etymology

The word reflexive is originally derived from the Medieval Latin reflexivus ('recoiling' [cf. reflex], or 'directed upon itself') (c. 1250 AD) from the classical Latin reflexus- ('turn away', 'reflection') + -īvus (suffix). The word entered Early Modern English in the 1580s. The sense of the word meaning 'directed upon itself', as now used in mathematics, surviving mostly by its use in philosophy and grammar (cf. Reflexive verb and Reflexive pronoun).

The first explicit use of "reflexivity", that is, describing a relation as having the property that every element is related to itself, is generally attributed to Giuseppe Peano in his Arithmetices principia (1889), wherein he defines one of the fundamental properties of equality being $a=a$ . The first use of the word reflexive in the sense of mathematics and logic was by Bertrand Russell in his Principles of Mathematics (1903).

02Definitions

A relation $R$ on the set $X$ is said to be reflexive if for every $x\in X$ , $(x,x)\in R$ .

Equivalently, letting $\operatorname {I} _{X}:=\{(x,x)~:~x\in X\}$ denote the identity relation on $X$ , the relation $R$ is reflexive if $\operatorname {I} _{X}\subseteq R$ .

The reflexive closure of $R$ is the union $R\cup \operatorname {I} _{X},$ which can equivalently be defined as the smallest (with respect to $\subseteq$ ) reflexive relation on $X$ that is a superset of $R.$ A relation $R$ is reflexive if and only if it is equal to its reflexive closure.

The reflexive reduction or irreflexive kernel of $R$ is the smallest (with respect to $\subseteq$ ) relation on $X$ that has the same reflexive closure as $R.$ It is equal to $R\setminus \operatorname {I} _{X}=\{(x,y)\in R~:~x\neq y\}.$ The reflexive reduction of $R$ can, in a sense, be seen as a construction that is the "opposite" of the reflexive closure of $R.$ For example, the reflexive closure of the canonical strict inequality $<$ on the reals $\mathbb {R}$ is the usual non-strict inequality $\leq$ whereas the reflexive reduction of $\leq$ is $<.$

Related definitions

There are several definitions related to the reflexive property. The relation $R$ is called:

irreflexive, anti-reflexive or aliorelative: if it does not relate any element to itself; that is, if $xRx$ holds for no $x\in X.$ A relation is irreflexive if and only if its complement in $X\times X$ is reflexive. An asymmetric relation is necessarily irreflexive. A transitive and irreflexive relation is necessarily asymmetric.
left quasi-reflexive: if whenever $x,y\in X$ are such that $xRy,$ then necessarily $xRx.$
right quasi-reflexive: if whenever $x,y\in X$ are such that $xRy,$ then necessarily $yRy.$
quasi-reflexive: if every element that is part of some relation is related to itself. Explicitly, this means that whenever $x,y\in X$ are such that $xRy,$ then necessarily $xRx$ and $yRy.$ Equivalently, a binary relation is quasi-reflexive if and only if it is both left quasi-reflexive and right quasi-reflexive. A relation $R$ is quasi-reflexive if and only if its symmetric closure $R\cup R^{\operatorname {T} }$ is left (or right) quasi-reflexive.
antisymmetric: if whenever $x,y\in X$ are such that $xRy{\text{ and }}yRx,$ then necessarily $x=y.$
coreflexive: if whenever $x,y\in X$ are such that $xRy,$ then necessarily $x=y.$ A relation $R$ is coreflexive if and only if its symmetric closure is anti-symmetric.

A reflexive relation on a nonempty set $X$ can neither be irreflexive, nor asymmetric ( $R$ is called asymmetric if $xRy$ implies not $yRx$ ), nor antitransitive ( $R$ is antitransitive if $xRy{\text{ and }}yRz$ implies not $xRz$ ).

03Examples

Examples of reflexive relations include:

"is equal to" (equality)
"is a subset of" (set inclusion)
"divides" (divisibility)
"is greater than or equal to"
"is less than or equal to"

Examples of irreflexive relations include:

"is not equal to"
"is coprime to" on the integers larger than 1
"is a proper subset of"
"is greater than"
"is less than"

An example of an irreflexive relation, which means that it does not relate any element to itself, is the "greater than" relation ( $x>y$ ) on the real numbers. Not every relation which is not reflexive is irreflexive; it is possible to define relations where some elements are related to themselves but others are not (that is, neither all nor none are). For example, the binary relation "the product of $x$ and $y$ is even" is reflexive on the set of even numbers, irreflexive on the set of odd numbers, and neither reflexive nor irreflexive on the set of natural numbers.

An example of a quasi-reflexive relation $R$ is "has the same limit as" on the set of sequences of real numbers: not every sequence has a limit, and thus the relation is not reflexive, but if a sequence has the same limit as some sequence, then it has the same limit as itself. An example of a left quasi-reflexive relation is a left Euclidean relation, which is always left quasi-reflexive but not necessarily right quasi-reflexive, and thus not necessarily quasi-reflexive.

An example of a coreflexive relation is the relation on integers in which each odd number is related to itself and there are no other relations. The equality relation is the only example of a both reflexive and coreflexive relation, and any coreflexive relation is a subset of the identity relation. The union of a coreflexive relation and a transitive relation on the same set is always transitive.

04Number of reflexive relations

The number of reflexive relations on an $n$ -element set is $2^{n^{2}-n}.$

Number of n-element binary relations of different types
Elements	Any	Transitive	Reflexive	Symmetric	Preorder	Partial order	Total preorder	Total order	Equivalence relation
0	1	1	1	1	1	1	1	1	1
1	2	2	1	2	1	1	1	1	1
2	16	13	4	8	4	3	3	2	2
3	512	171	64	64	29	19	13	6	5
4	65,536	3,994	4,096	1,024	355	219	75	24	15
n	2^n²		2ⁿ⁽ⁿ⁻¹⁾	2^n(n+1)/2			∑ⁿ _k=0 k!S(n, k)	n!	∑ⁿ _k=0 S(n, k)
OEIS	A002416	A006905	A053763	A006125	A000798	A001035	A000670	A000142	A000110

Note that S(n, k) refers to Stirling numbers of the second kind.

05Philosophical logic

Authors in philosophical logic often use different terminology. Reflexive relations in the mathematical sense are called totally reflexive in philosophical logic, and quasi-reflexive relations are called reflexive.

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Sources and credits

This article is adapted from the Wikipedia article “Reflexive relation”, written by its contributors and licensed under CC BY-SA 4.0. Fathomly has changed the layout, removed citation markers, navigation and maintenance notices, and adjusted punctuation. This adapted version is shared under the same license. For references, see the original article.

Images, from Wikimedia Commons:

Arithmetices principia Properties of equality.png by Giuseppe Peano, Public domain

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