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Separation relation : ウィキペディア英語版
Separation relation
In mathematics, a separation relation is a formal way to arrange a set of objects in an unoriented circle. It is defined as a quaternary relation ''S''(''a'', ''b'', ''c'', ''d'') satisfying certain axioms, which is interpreted as asserting that ''a'' and ''c'' separate ''b'' from ''d''.
Whereas a linear order endows a set with a positive end and a negative end, a separation relation forgets not only which end is which, but also where the ends are. In this way it is a final, further weakening of the concepts of a betweenness relation and a cyclic order. There is generally nothing else that can be forgotten: up to the relevant sense of interdefinability, these three relations are the only nontrivial reducts of the ordered set of rational numbers.
==Application==
The separation may be used in showing the real projective plane is a complete space. The separation relation was described with axioms in 1898 by Giovanni Vailati.〔Bertrand Russell (1903) Principles of Mathematics, page 214〕
* ''abcd'' = ''badc''
* ''abcd'' = ''adcb''
* ''abcd'' ⇒ ¬ ''acbd''
* ''abcd'' ∨ ''acdb'' ∨ ''adbc''
* ''abcd'' ∧ ''acde'' ⇒ ''abde''.
The relation of separation of points was written AC//BD by H. S. M. Coxeter in his textbook ''The Real Projective Plane''.〔H. S. M. Coxeter (1949) ''The Real Projective Plane'', Chapter 10: Continuity, McGraw Hill〕 The axiom of continuity used is "Every monotonic sequence of points has a limit." The separation relation is used to provide definitions:
* is monotonic ≡ ∀ ''n'' > 1 A_0 A_n // A_1 A_.
* ''M'' is a limit ≡ (∀ ''n'' > 2 A_1 A_n // A_2 M) ∧ (∀ P A_1P // A_2 M ⇒ ∃ ''n'' A_1 A_n // P M ).

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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