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Easton's theorem : ウィキペディア英語版
Easton's theorem
In set theory, Easton's theorem is a result on the possible cardinal numbers of powersets. (extending a result of Robert M. Solovay) showed via forcing that
: \kappa < \operatorname(2^\kappa)\,
and, for \kappa < \lambda\,, that
: 2^\kappa\le 2^\lambda\,
are the only constraints on permissible values for 2κ when κ is a regular cardinal.
== Statement of the theorem ==

Easton's theorem states that if ''G'' is a class function whose domain consists of ordinals and whose range consists of ordinals such that
# ''G'' is non-decreasing,
# the cofinality of \aleph_ is greater than \aleph_ for each α in the domain of G, and
# \aleph_ is regular for each α in the domain of G,
then there is a model of ZFC such that
:2^\,
for each \alpha in the domain of ''G''.
The proof of Easton's theorem uses forcing with a proper class of forcing conditions over a model satisfying the generalized continuum hypothesis.
The first two conditions in the theorem are necessary. Condition 1 is a well known property of cardinality, while condition 2 follows from König's theorem.
In Easton's model the powersets of singular cardinals have the smallest possible cardinality compatible with the conditions that 2κ has cofinality greater than κ and is a non-decreasing function of κ.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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