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{{more citations needed|date=October 2013}}
In [[mathematics]], a [[binary relation]] {{math|''R''}} over a [[Set (mathematics)|set]] {{math|''X''}} is '''transitive''' if for all elements {{math|''a''}}, {{math|''b''}}, {{math|''c''}} in {{math|''X''}}, whenever {{math|''a''}} is related to {{math|''b''}} and {{math|''b''}} is related to {{math|''c''}}, then {{math|''a''}} is also related to {{math|''c''}}. Transitivity is a key property of both [[partial order]]s and [[equivalence relation]]s.
== Definition ==
{{stack|{{Binary relations}}}}
A binary relation {{mvar|R}} on the set {{mvar|X}} is a ''transitive relation'' if,<ref>{{harvnb|Smith|Eggen|St. Andre|2006|loc=p. 145}}</ref>
:for all {{math|''a'', ''b'', ''c'' ∈ ''X''}}, if {{math|''a R b''}} and {{math|''b R c''}}, then {{math|''a R c''}}.
Or in terms of [[first-order logic]]:
:<math>\forall a,b,c \in X: (aRb \wedge bRc) \Rightarrow aRc,</math>
where {{math|''a R b''}} is the [[infix notation]] for {{math|(''a'', ''b'') ∈ ''R''}}.

==Examples==
As a nonmathematical example, the relation "is an ancestor of" is transitive. For example, if Amy is an ancestor of Becky, and Becky is an ancestor of Carrie, then Amy, too, is an ancestor of Carrie.

On the other hand, "is the birth parent of" is not a transitive relation, because if Alice is the birth parent of Brenda, and Brenda is the birth parent of Claire, then Alice is not the birth parent of Claire. What is more, it is [[antitransitive]]: Alice can ''never'' be the birth parent of Claire.

"Is greater than", "is at least as great as", and "is equal to" ([[equality (mathematics)|equality]]) are transitive relations on various sets, for instance, the set of real numbers or the set of natural numbers:

: whenever ''x'' &gt; ''y'' and ''y'' &gt; ''z'', then also ''x'' &gt; ''z''
: whenever ''x'' &ge; ''y'' and ''y'' &ge; ''z'', then also ''x'' &ge; ''z''
: whenever ''x'' = ''y'' and ''y'' = ''z'', then also ''x'' = ''z''.

More examples of transitive relations:
* "is a [[subset]] of" (set inclusion, a relation on sets)
* "divides" ([[divisor|divisibility]], a relation on natural numbers)
* "implies" ([[material conditional|implication]], symbolized by "⇒", a relation on [[proposition]]s)
Examples of non-transitive relations:
* "is the [[successor function|successor]] of" (a relation on natural numbers)
* "is a member of the set" (symbolized as "∈")<ref>However, the class of [[von Neumann ordinal]]s is constructed in a way such that ∈ ''is'' transitive when restricted to that class.</ref>
* "is [[perpendicular]] to" (a relation on lines in [[Euclidean geometry]])

The [[empty relation]] on any set <math>X</math> is transitive<ref>{{harvnb|Smith|Eggen|St. Andre|2006|loc=p. 146}}</ref><ref>https://courses.engr.illinois.edu/cs173/sp2011/Lectures/relations.pdf</ref> because there are no elements <math>a,b,c \in X</math> such that <math>aRb</math> and <math>bRc</math>, and hence the transitivity condition is [[vacuous truth|vacuously true]]. A relation {{math|''R''}} containing only one [[ordered pair]] is also transitive: if the ordered pair is of the form <math>(x, x)</math> for some <math>x \in X</math> the only such elements <math>a,b,c \in X</math> are <math>a=b=c=x</math>, and indeed in this case <math>aRc</math>, while if the ordered pair is not of the form <math>(x, x)</math> then there are no such elements <math>a,b,c \in X</math> and hence <math>R</math> is vacuously transitive.

== Properties ==

=== Closure properties ===
* The [[inverse relation|inverse]] (converse) of a transitive relation is always transitive. For instance, knowing that "is a [[subset]] of" is transitive and "is a [[superset]] of" is its inverse, one can conclude that the latter is transitive as well.
* The intersection of two transitive relations is always transitive. For instance, knowing that "was born before" and "has the same first name as" are transitive, one can conclude that "was born before and also has the same first name as" is also transitive.
* The union of two transitive relations need not be transitive. For instance, "was born before or has the same first name as" is not a transitive relation, since e.g. [[Herbert Hoover]] is related to [[Franklin D. Roosevelt]], which is in turn related to [[Franklin Pierce]], while Hoover is not related to Franklin Pierce.
* The complement of a transitive relation need not be transitive. For instance, while "equal to" is transitive, "not equal to" is only transitive on sets with at most one element.

=== Other properties ===
A transitive relation is [[asymmetric relation|asymmetric]] if and only if it is [[irreflexive relation|irreflexive]].<ref>{{cite book|last1=Flaška|first1=V.|last2=Ježek|first2=J.|last3=Kepka|first3=T.|last4=Kortelainen|first4=J.|title=Transitive Closures of Binary Relations I|year=2007|publisher=School of Mathematics - Physics Charles University|location=Prague|page=1|url=http://www.karlin.mff.cuni.cz/~jezek/120/transitive1.pdf|url-status=dead|archiveurl=https://web.archive.org/web/20131102214049/http://www.karlin.mff.cuni.cz/~jezek/120/transitive1.pdf|archivedate=2013-11-02}} Lemma 1.1 (iv). Note that this source refers to asymmetric relations as "strictly antisymmetric".</ref>

A transitive relation need not be [[Reflexive relation|reflexive]]. When it is, it is called a [[preorder]]. For example, on set ''X'' = {1,2,3}:

* ''R'' = {{{Hair space}}(1,1), (2,2), (3,3), (1,3), (3,2){{Hair space}}} is reflexive, but not transitive, as the pair (1,2) is absent,
* ''R'' = {{{Hair space}}(1,1), (2,2), (3,3), (1,3){{Hair space}}} is reflexive as well as transitive, so it is a preorder,
* ''R'' = {{{Hair space}}(1,1), (2,2), (3,3){{Hair space}}} is reflexive as well as transitive, another preorder.

==Transitive extensions and transitive closure==
{{main|Transitive closure}}

Let {{mvar|R}} be a binary relation on set {{mvar|X}}. The ''transitive extension'' of {{mvar|R}}, denoted {{math|''R''<sub>1</sub>}}, is the smallest binary relation on {{mvar|X}} such that {{math|''R''<sub>1</sub>}} contains {{mvar|R}}, and if {{math|(''a'', ''b'') ∈ ''R''}} and {{math|(''b'', ''c'') ∈ ''R''}} then {{math|(''a'', ''c'') ∈ ''R''<sub>1</sub>}}.<ref>{{harvnb|Liu|1985|loc=p. 111}}</ref> For example, suppose {{mvar|X}} is a set of towns, some of which are connected by roads. Let {{mvar|R}} be the relation on towns where {{math|(''A'', ''B'') ∈ ''R''}} if there is a road directly linking town {{mvar|A}} and town {{mvar|B}}. This relation need not be transitive. The transitive extension of this relation can be defined by {{math|(''A'', ''C'') ∈ ''R''<sub>1</sub>}} if you can travel between towns {{mvar|A}} and {{mvar|C}} by using at most two roads.

If a relation is transitive then its transitive extension is itself, that is, if {{mvar|R}} is a transitive relation then {{math|1=''R''<sub>1</sub> = ''R''}}.

The transitive extension of {{math|''R''<sub>1</sub>}} would be denoted by {{math|''R''<sub>2</sub>}}, and continuing in this way, in general, the transitive extension of {{math|''R''<sub>''i''</sub>}} would be {{math|''R''<sub>''i'' + 1</sub>}}. The ''transitive closure'' of {{mvar|R}}, denoted by {{math|''R''*}} or {{math|''R''<sup>∞</sup>}} is the set union of {{mvar|R}}, {{math|''R''<sub>1</sub>}}, {{math|''R''<sub>2</sub>}}, ... .<ref name=Liu112>{{harvnb|Liu|1985|loc=p. 112}}</ref>

The transitive closure of a relation is a transitive relation.<ref name=Liu112 />

The relation "is the birth parent of" on a set of people is not a transitive relation. However, in biology the need often arises to consider birth parenthood over an arbitrary number of generations: the relation "is a birth ancestor of" ''is'' a transitive relation and it is the transitive closure of the relation "is the birth parent of".

For the example of towns and roads above, {{math|(''A'', ''C'') ∈ ''R''*}} provided you can travel between towns {{mvar|A}} and {{mvar|C}} using any number of roads.

== Relation properties that require transitivity ==
* [[Preorder]] – a [[reflexive relation|reflexive]] transitive relation
* [[Partially ordered set|Partial order]] – an [[antisymmetric relation|antisymmetric]] preorder
* [[Total preorder]] – a [[total relation|total]] preorder
* [[Equivalence relation]] – a [[symmetric relation|symmetric]] preorder
* [[Strict weak ordering]] – a strict partial order in which incomparability is an equivalence relation
* [[Total ordering]] – a [[total relation|total]], [[antisymmetric relation|antisymmetric]] transitive relation

==Counting transitive relations==

No general formula that counts the number of transitive relations on a finite set {{OEIS|id=A006905}} is known.<ref>Steven R. Finch, [http://www.people.fas.harvard.edu/~sfinch/csolve/posets.pdf "Transitive relations, topologies and partial orders"], 2003.</ref> However, there is a formula for finding the number of relations that are simultaneously reflexive, symmetric, and transitive – in other words, [[equivalence relation]]s – {{OEIS|id=A000110}}, those that are symmetric and transitive, those that are symmetric, transitive, and antisymmetric, and those that are total, transitive, and antisymmetric. Pfeiffer<ref>Götz Pfeiffer, "[http://www.cs.uwaterloo.ca/journals/JIS/VOL7/Pfeiffer/pfeiffer6.html Counting Transitive Relations]", ''Journal of Integer Sequences'', Vol. 7 (2004), Article 04.3.2.</ref> has made some progress in this direction, expressing relations with combinations of these properties in terms of each other, but still calculating any one is difficult. See also.<ref>Gunnar Brinkmann and Brendan D. McKay,"[http://cs.anu.edu.au/~bdm/papers/topologies.pdf Counting unlabelled topologies and transitive relations]"</ref>

{{number of relations}}

== Related properties ==
[[File:Rock-paper-scissors.svg|alt=Cycle diagram|thumb|The [[Rock–paper–scissors]] game is based on an intransitive and antitransitive relation "''x'' beats ''y''".]]
A relation ''R'' is called ''[[intransitivity|intransitive]]'' if it is not transitive, that is, if ''xRy'' and ''yRz'', but not ''xRz'', for some ''x'', ''y'', ''z''.
In contrast, a relation ''R'' is called ''[[antitransitive]]'' if ''xRy'' and ''yRz'' always implies that ''xRz'' does not hold.
For example, the relation defined by ''xRy'' if ''xy'' is an [[even number]] is intransitive,<ref>since e.g. 3''R''4 and 4''R''5, but not 3''R''5</ref> but not antitransitive.<ref>since e.g. 2''R''3 and 3''R''4 and 2''R''4</ref> The relation defined by ''xRy'' if ''x'' is even and ''y'' is [[odd number|odd]] is both transitive and antitransitive.<ref>since ''xRy'' and ''yRz'' can never happen</ref> 
The relation defined by ''xRy'' if ''x'' is the [[successor function|successor]] number of ''y'' is both intransitive<ref>since e.g. 3''R''2 and 2''R''1, but not 3''R''1</ref> and antitransitive.<ref>since, more generally, ''xRy'' and ''yRz'' implies ''x''=''y''+1=''z''+2≠''z''+1, i.e. not ''xRz'', for all ''x'', ''y'', ''z''</ref> Unexpected examples of intransitivity arise in situations such as political questions or group preferences.<ref>{{Cite news|url=https://www.motherjones.com/kevin-drum/2018/11/preferences-are-not-transitive/|title=Preferences are not transitive|last=Drum|first=Kevin|date=November 2018|work=Mother Jones|access-date=2018-11-29}}</ref>

Generalized to stochastic versions (''[[stochastic transitivity]]''), the study of transitivity finds applications of in [[decision theory]], [[psychometrics]] and [[Utilitarianism|utility models]].<ref>{{Cite journal|last=Oliveira|first=I.F.D.|last2=Zehavi|first2=S.|last3=Davidov|first3=O.|date=August 2018|title=Stochastic transitivity: Axioms and models|url=http://dx.doi.org/10.1016/j.jmp.2018.06.002|journal=Journal of Mathematical Psychology|volume=85|pages=25–35|doi=10.1016/j.jmp.2018.06.002|issn=0022-2496}}</ref>

A ''[[quasitransitive relation]]'' is another generalization; it is required to be transitive only on its non-symmetric part. Such relations are used in [[social choice theory]] or [[microeconomics]].<ref>{{cite journal | last=Sen | first=A. | authorlink=Amartya Sen | title=Quasi-transitivity, rational choice and collective decisions | zbl=0181.47302 | journal=Rev. Econ. Stud. | volume=36 | pages=381–393 | year=1969 | doi=10.2307/2296434 | ref=harv }}</ref>

==See also==

* [[Transitive reduction]]
* [[Nontransitive dice]]
* [[Rational choice theory#Formal statement|Rational choice theory]]

== Notes ==
{{reflist}}

== References ==
* {{citation|first=Ralph P.|last=Grimaldi|authorlink=Ralph Grimaldi|title=Discrete and Combinatorial Mathematics|year=1994|publisher=Addison-Wesley|edition=3rd|isbn=0-201-19912-2}}
* {{citation|first=C.L.|last=Liu|title=Elements of Discrete Mathematics|year=1985|publisher=McGraw-Hill|isbn=0-07-038133-X|url-access=registration|url=https://archive.org/details/elementsofdiscre00liuc}}
*[[Gunther Schmidt]], 2010. ''Relational Mathematics''. Cambridge University Press, {{isbn|978-0-521-76268-7}}.
* {{citation|first1=Douglas|last1=Smith|first2=Maurice|last2=Eggen|first3=Richard|last3=St.Andre|title=A Transition to Advanced Mathematics|edition=6th|year=2006|publisher=Brooks/Cole|isbn=978-0-534-39900-9}}

==External links==
* {{springer|title=Transitivity|id=p/t093810}}
* [http://www.cut-the-knot.org/triangle/remarkable.shtml Transitivity in Action] at [[cut-the-knot]]

[[Category:Transitive relations| ]]
[[Category:Elementary algebra]]