The fact that an element x belongs to a set 5 will be denoted by xeS (not by xC S), while TC S will mean that the set T is a subset of the set S. Other set-theoretical notations will be used : the sum of a set (S, T, • • • ) of sets is@(5, T, ■ • • ), the product (that is, the common part of the elements) of (S, T, ■ ■ • ) is^S, T, • • • ), and the complementary set to S is65.(These are not the notations of [1].
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John von Neumann (1935) studied this question.