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Wigner-d'Espagnat inequality
The '''Wigner - d'Espagnat inequality''' is a basic result of [[set theory]].
It is named for [[Eugene Wigner]] and [[Bertrannard d'Espagnat]] who (as pointed out by [[John Stewart Bell|Bell]]) both employed it in their popularizations of [[quantum mechanics]].

Given a set S with three subsets, J, K, and L, the following holds:

* each member of S which is a member of J, but not of L 
:: is either a member of J, but neither of K, nor of L, 
:: or else is a member of J and of K, but not of L;
* each member of J which is neither a member of K, nor of L, is therefore a member of J, but not of K; and
(contracted; show full)

==Reference==
* John S. Bell, ''Bertlmann's socks and the nature of reality'', Journal de Physique '''42''', no. 3, p. 41 (1981); and references therein.

[[Category:Inequalities]]