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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 Bertrand 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, 
(contracted; show full)

Similar interdependencies between ''two'' particular measurements and the corresponding operators are the [[Robertson - Schrödinger relation|Uncertainty relations]] a
res first expressed by [[Werner Heisenberg|Heisenberg]] for the interdependence between measurements of distance and of momentum, and as generalized by [[Edward Condon]], [[Howard Percy Robertson]], and [[Erwin Schrödinger]].

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