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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Singular cardinals hypothesis ： ウィキペディア英語版 Singular cardinals hypothesis In set theory, the singular cardinals hypothesis (SCH) arose from the question of whether the least cardinal number for which the generalized continuum hypothesis (GCH) might fail could be a singular cardinal. According to Mitchell (1992), the singular cardinals hypothesis is: :If κ is any singular strong limit cardinal, then 2κ = κ+. Here, κ+ denotes the successor cardinal of κ. Since SCH is a consequence of GCH which is known to be consistent with ZFC, SCH is consistent with ZFC. The negation of SCH has also been shown to be consistent with ZFC, if one assumes the existence of a sufficiently large cardinal number. In fact, by results of Moti Gitik, ZFC + the negation of SCH is equiconsistent with ZFC + the existence of a measurable cardinal κ of Mitchell order κ++. Another form of the SCH is the following statement: :2cf(κ) < κ implies κcf(κ) = κ+, where cf denotes the cofinality function. Note that κcf(κ)= 2κ for all singular strong limit cardinals κ. The second formulation of SCH is strictly stronger than the first version, since the first one only mentions strong limits; from a model in which the first version of SCH fails at ℵω and GCH holds above ℵω+2, we can construct a model in which the first version of SCH holds but the second version of SCH fails, by adding ℵω Cohen subsets to ℵn for some n. Silver proved that if κ is singular with uncountable cofinality and 2λ = λ+ for all infinite cardinals λ < κ, then 2κ = κ+. Silver's original proof used generic ultrapowers. The following important fact follows from Silver's theorem: if the singular cardinals hypothesis holds for all singular cardinals of countable cofinality, then it holds for all singular cardinals. In particular, then, if $\backslash kappa$ is the least counterexample to the singular cardinals hypothesis, then $cf(\backslash kappa)\; =\; \backslash omega$. The negation of the singular cardinals hypothesis is intimately related to violating the GCH at a measurable cardinal. A well-known result of Dana Scott is that if the GCH holds below a measurable cardinal $\backslash kappa$ on a set of measure one—i.e., there is normal $\backslash kappa$ -complete ultrafilter D on $\backslash mathcal(\backslash kappa)$ such that $\backslash \backslash in\; D$, then $2^\backslash kappa\; =\; \backslash kappa^+$. Starting with $\backslash kappa$ a supercompact cardinal, Silver was able to produce a model of set theory in which $\backslash kappa$ is measurable and in which $2^\backslash kappa\; >\; \backslash kappa^+$. Then, by applying Prikry forcing to the measurable $\backslash kappa$, one gets a model of set theory in which $\backslash kappa$ is a strong limit cardinal of countable cofinality and in which $2^\backslash kappa\; >\; \backslash kappa^+$—a violation of the SCH. Gitik, building on work of Woodin, was able to replace the supercompact in Silver's proof with a measurable of Mitchell order $\backslash kappa^$. That established an upper bound for the consistency strength of the failure of the SCH. Gitik again, using results of Inner model theory, was able to show that a measurable of Mitchell order $\backslash kappa^$ is also the lower bound for the consistency strength of the failure of SCH. A wide variety of propositions imply SCH. As was noted above, GCH implies SCH. On the other hand, the proper forcing axiom which implies $2^\; =\; \backslash aleph\_2$ and hence is incompatible with GCH also implies SCH. Solovay showed that large cardinals almost imply SCH—in particular, if $\backslash kappa$ is strongly compact cardinal, then the SCH holds above $\backslash kappa$. On the other hand, the non-existence of (inner models for) various large cardinals (below a measurable of Mitchell order $\backslash kappa^$) also imply SCH. ==References== * T. Jech: (Properties of the gimel function and a classification of singular cardinals ), Fundamenta Mathematicae 81 (1974): 57-64. * William J. Mitchell, "On the singular cardinal hypothesis," ''Trans. Amer. Math. Soc.'', volume 329 (2): pp. 507–530, 1992. * Jason Aubrey, ''The Singular Cardinals Problem'' ((PDF )), VIGRE expository report, Department of Mathematics, University of Michigan. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Singular cardinals hypothesis」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.