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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 Limit point compact ： ウィキペディア英語版 Limit point compact In mathematics, a topological space ''X'' is said to be limit point compact〔The terminology "limit point compact" appears in a topology textbook by James Munkres, and is apparently due to him. According to him, some call the property "Fréchet compactness", while others call it the "Bolzano-Weierstrass property". Munkres, p. 178–179.〕 or weakly countably compact if every infinite subset of ''X'' has a limit point in ''X''. This property generalizes a property of compact spaces. In a metric space, limit point compactness, compactness, and sequential compactness are all equivalent. For general topological spaces, however, these three notions of compactness are not equivalent. ==Properties and Examples== * Limit point compactness is equivalent to countable compactness if ''X'' is a T1-space and is equivalent to compactness if ''X'' is a metric space. * An example of a space ''X'' that is not weakly countably compact is any countable (or larger) set with the discrete topology. A more interesting example is the countable complement topology. * Even though a continuous function from a compact space ''X'', to an ordered set ''Y'' in the order topology, must be bounded, the same thing does not hold if ''X'' is ''limit point compact''. An example is given by the space $X\backslash times\backslash mathbb$ (where ''X'' = carries the indiscrete topology and $\backslash mathbb$ is the set of all integers carrying the discrete topology) and the function $f=\backslash pi\_$ is limit point compact (in fact, ''every'' nonempty subset of $X\backslash times\; \backslash mathbb$ has a limit point) but ƒ is not bounded, and in fact $f(X\backslash times\; \backslash mathbb)=\backslash mathbb$ is not even limit point compact. * Every countably compact space (and hence every compact space) is weakly countably compact, but the converse is not true. * For metrizable spaces, compactness, limit point compactness, and sequential compactness are all equivalent. * The set of all real numbers, R, is not limit point compact; the integers are an infinite set but do not have a limit point in R. *If (''X'', ''T'') and (''X'', ''T *'') are topological spaces with ''T *'' finer than ''T'' and (''X'', ''T *'') is limit point compact, then so is (''X'', ''T''). *A finite space is vacuously limit point compact. 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「Limit point compact」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.