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Cardinal characteristic of the continuum : ウィキペディア英語版 | Cardinal characteristic of the continuum In the mathematical discipline of set theory, a cardinal characteristic of the continuum is an infinite cardinal number that may consistently lie strictly between (the cardinality of the set of natural numbers), and the cardinality of the continuum, that is, the cardinality of the set of all real numbers. The latter cardinal is denoted or . A variety of such cardinal characteristics arise naturally, and much work has been done in determining what relations between them are provable, and constructing models of set theory for various consistent configurations of them. == Background ==
Cantor's diagonal argument shows that is strictly greater than , but it does not specify whether it is the ''least'' cardinal greater than (that is, ). Indeed the assumption that is the well-known Continuum Hypothesis, which was shown to be independent of the standard ZFC axioms for set theory by Paul Cohen. If the Continuum Hypothesis fails and so is at least , natural questions arise about the cardinals strictly between and , for example regarding Lebesgue measurability. By considering the least cardinal with some property, one may get a definition for an uncountable cardinal that is consistently less than . Generally one only considers definitions for cardinals that are provably greater than and at most as cardinal characteristics of the continuum, so if the Continuum Hypothesis holds they are all equal to .
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