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Relation composition : ウィキペディア英語版
Composition of relations
In mathematics, the composition of binary relations is a concept of forming a new relation from two given relations ''R'' and ''S'', having as its most well-known special case the composition of functions.
== Definition ==
If R\subseteq X\times Y and S\subseteq Y\times Z are two binary relations, then
their composition S\circ R is the relation
:S\circ R = \.
In other words, S\circ R\subseteq X\times Z is defined by the rule that says (x,z)\in S\circ R if and only if there is an element y\in Y such that x\,R\,y\,S\,z (i.e. (x,y)\in R and (y,z)\in S).
In particular fields, authors might denote by what is defined here to be .
The convention chosen here is such that function composition (with the usual notation) is obtained as a special case, when ''R'' and ''S'' are functional relations. Some authors〔Kilp, Knauer & Mikhalev, p. 7〕 prefer to write \circ_l and \circ_r explicitly when necessary, depending whether the left or the right relation is the first one applied.
A further variation encountered in computer science is the Z notation: \circ is used to denote the traditional (right) composition, but ⨾ ; (a fat open semicolon with Unicode code point U+2A3E) denotes left composition.〔ISO/IEC 13568:2002(E), p. 23〕〔http://www.fileformat.info/info/unicode/char/2a3e/index.htm〕 This use of semicolon coincides with the notation for function composition used (mostly by computer scientists) in Category theory,〔http://www.math.mcgill.ca/triples/Barr-Wells-ctcs.pdf, p. 6〕 as well as the notation for dynamic conjunction within linguistic dynamic semantics.〔http://plato.stanford.edu/entries/dynamic-semantics/#EncDynTypLog〕 The semicolon notation (with this semantic) was introduced by Ernst Schröder in 1895.〔 A free HTML version of the book is available at http://www.cs.man.ac.uk/~pt/Practical_Foundations/〕
The binary relations R\subseteq X\times Y are sometimes regarded as the morphisms R\colon X\to Y in a category Rel which has the sets as objects. In Rel, composition of morphisms is exactly composition of relations as defined above. The category Set of sets is a subcategory of Rel that has the same objects but fewer morphisms. A generalization of this is found in the theory of allegories.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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