
1) Basic concept in set theory
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https://www.crosswordclues.com/clue/universal-set

In set theory, a universal set is a set which contains all objects, including itself. In set theory as usually formulated, the conception of a universal set leads to a paradox (Russell`s paradox) and is consequently not allowed. However, some non-standard variants of set theory include a universal set. ==Reasons for nonexistence== Zermelo–Fraen....
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http://en.wikipedia.org/wiki/Universal_set

(from the article `logic, history of`) Boole used capital letters to stand for the extensions of terms; they are referred to (in 1854) as classes of `things` but should not be understood ... ..., is the class of all objects that are members of but not of i.e., { : }; the universal class, symbolized as V, is the cl...
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http://www.britannica.com/eb/a-z/u/13
noun (mathematics) the set that contains all the elements or objects involved in the problem under consideration; `all other sets are subsets of the universal set`
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https://www.encyclo.co.uk/local/20974
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