In

Encyclopedia Britannica

calls this property quasi-reflexivity. ;: If whenever $x,\; y\; \backslash in\; X$ are such that $x\; R\; y,$ then necessarily $y\; R\; y.$ ;: If every element that is part of some relation is related to itself. Explicitly, this means that whenever $x,\; y\; \backslash in\; X$ are such that $x\; R\; y,$ then necessarily $x\; R\; x$ $y\; R\; y.$ 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 $R\; \backslash cup\; R^$ is left (or right) quasi-reflexive. ; Antisymmetric: If whenever $x,\; y\; \backslash in\; X$ are such that $x\; R\; y\; \backslash text\; y\; R\; x,$ then necessarily $x\; =\; y.$ ;: If whenever $x,\; y\; \backslash in\; X$ are such that $x\; R\; y,$ 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 if $x\; R\; y$ implies not $y\; R\; x$), nor antitransitive ($R$ is if $x\; R\; y\; \backslash text\; y\; R\; z$ implies not $x\; R\; z$).

mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern 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 number
In mathematics, a real number is a number that can be used to measurement, measure a ''continuous'' one-dimensional quantity such as a distance, time, duration or temperature. Here, ''continuous'' means that values can have arbitrarily small var ...

s, 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
Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definiti ...

and transitivity, reflexivity is one of three properties defining equivalence relations.
Definitions

Let $R$ be a binary relation on a set $X,$ which by definition is just a subset of $X\; \backslash times\; X.$ For any $x,\; y\; \backslash in\; X,$ the notation $x\; R\; y$ means that $(x,\; y)\; \backslash in\; R$ while "not $x\; R\; y$" means that $(x,\; y)\; \backslash not\backslash in\; R.$ The relation $R$ is called if $x\; R\; x$ for every $x\; \backslash in\; X$ or equivalently, if $\backslash operatorname\_X\; \backslash subseteq\; R$ where $\backslash operatorname\_X\; :=\; \backslash $ denotes the identity relation on $X.$ The of $R$ is the union $R\; \backslash cup\; \backslash operatorname\_X,$ which can equivalently be defined as the smallest (with respect to $\backslash subseteq$) 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 or of $R$ is the smallest (with respect to $\backslash subseteq$) relation on $X$ that has the same reflexive closure as $R.$ It is equal to $R\; \backslash setminus\; \backslash operatorname\_X\; =\; \backslash .$ The irreflexive kernel 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 $\backslash mathbb$ is the usual non-strict inequality $\backslash leq$ whereas the reflexive reduction of $\backslash leq$ is $<.$Related definitions

There are several definitions related to the reflexive property. The relation $R$ is called: ;, or : If it does not relate any element to itself; that is, if not $x\; R\; x$ for every $x\; \backslash in\; X.$ A relation is irreflexive if and only if its complement in $X\; \backslash times\; X$ is reflexive. An asymmetric relation is necessarily irreflexive. A transitive and irreflexive relation is necessarily asymmetric. ;: If whenever $x,\; y\; \backslash in\; X$ are such that $x\; R\; y,$ then necessarily $x\; R\; x.$ThEncyclopedia Britannica

calls this property quasi-reflexivity. ;: If whenever $x,\; y\; \backslash in\; X$ are such that $x\; R\; y,$ then necessarily $y\; R\; y.$ ;: If every element that is part of some relation is related to itself. Explicitly, this means that whenever $x,\; y\; \backslash in\; X$ are such that $x\; R\; y,$ then necessarily $x\; R\; x$ $y\; R\; y.$ 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 $R\; \backslash cup\; R^$ is left (or right) quasi-reflexive. ; Antisymmetric: If whenever $x,\; y\; \backslash in\; X$ are such that $x\; R\; y\; \backslash text\; y\; R\; x,$ then necessarily $x\; =\; y.$ ;: If whenever $x,\; y\; \backslash in\; X$ are such that $x\; R\; y,$ 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 if $x\; R\; y$ implies not $y\; R\; x$), nor antitransitive ($R$ is if $x\; R\; y\; \backslash text\; y\; R\; z$ implies not $x\; R\; z$).

Examples

Examples of reflexive relations include: * "is equal to" ( equality) * "is asubset
In mathematics, Set (mathematics), set ''A'' is a subset of a set ''B'' if all Element (mathematics), elements of ''A'' are also elements of ''B''; ''B'' is then a superset of ''A''. It is possible for ''A'' and ''B'' to be equal; if they are ...

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 number
In mathematics, a real number is a number that can be used to measurement, measure a ''continuous'' one-dimensional quantity such as a distance, time, duration or temperature. Here, ''continuous'' means that values can have arbitrarily small var ...

s. 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 number
In mathematics, the natural numbers are those numbers used for counting (as in "there are ''six'' coins on the table") and ordering (as in "this is the ''third'' largest city in the country").
Numbers used for counting are called ''Cardinal n ...

s.
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 integer
An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign (−1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language of ...

s 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.
Number of reflexive relations

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

Authors inphilosophical logic
Understood in a narrow sense, philosophical logic is the area of logic that studies the application of logical methods to philosophy, philosophical problems, often in the form of extended logical systems like modal logic. Some theorists conceive ph ...

often use different terminology.
Reflexive relations in the mathematical sense are called totally reflexive in philosophical logic, and quasi-reflexive relations are called reflexive. Here: p.187
Notes

References

* Levy, A. (1979) ''Basic Set Theory'', Perspectives in Mathematical Logic, Springer-Verlag. Reprinted 2002, Dover. * Lidl, R. and Pilz, G. (1998). ''Applied abstract algebra'', Undergraduate Texts in Mathematics, Springer-Verlag. * Quine, W. V. (1951). ''Mathematical Logic'', Revised Edition. Reprinted 2003, Harvard University Press. * Gunther Schmidt, 2010. ''Relational Mathematics''. Cambridge University Press, .External links

* {{springer, title=Reflexivity, id=p/r080590 Binary relations