Easton's theorem
Mathematic theorem / From Wikipedia, the free encyclopedia
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In set theory, Easton's theorem is a result on the possible cardinal numbers of powersets. Easton (1970) (extending a result of Robert M. Solovay) showed via forcing that the only constraints on permissible values for 2κ when κ is a regular cardinal are
(where cf(α) is the cofinality of α) and