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・ Sepia stingray
・ Sepia subplana
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・ Sepia tanybracheia
・ Sepia Tears ~midwinter's reprise~
・ Separative work units
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・ Separatory funnel
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・ Separdan, Siahkal
Separoid
・ Separowo
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・ Separuh Aku
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Separoid : ウィキペディア英語版
Separoid

In mathematics, a separoid is a binary relation between disjoint sets which is stable as an ideal in the canonical order induced by inclusion. Many mathematical objects which appear to be quite different, find a common generalisation in the framework of separoids; e.g., graphs, configurations of convex sets, oriented matroids, and polytopes. Any countable category is an induced subcategory of separoids when they are endowed with homomorphisms () (viz., mappings that preserve the so-called ''minimal Radon partitions'').
In this general framework, some results and invariants of different categories turn out to be special cases of the same aspect; e.g., the pseudoachromatic number from graph theory and the Tverberg theorem from combinatorial convexity are simply two faces of the same aspect, namely, complete colouring of separoids.
== The axioms ==

A separoid () is a set S endowed with a binary relation \mid\ \subseteq2^S\times2^S on its power set, which satisfies the following simple properties for A,B\subseteq S:
: A\mid B\Leftrightarrow B\mid A,
: A\mid B\Rightarrow A\cap B=\varnothing,
: A\mid B \hbox A'\subset A\Rightarrow A'\mid B.
A related pair A\mid B is called a separation and we often say that ''A is separated from B''. It is enough to know the ''maximal'' separations to reconstruct the separoid.
A mapping \varphi\colon S\to T is a morphism of separoids if the preimages of separations are separations; that is, for A,B\subseteq T
: A\mid B\Rightarrow\varphi^(A)\mid\varphi^(B).

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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