{"title":"以$\\omega^{\\omega}$为基的拓扑空间","authors":"T. Banakh","doi":"10.4064/dm762-4-2018","DOIUrl":null,"url":null,"abstract":"Given a partially ordered set $P$ we study properties of topological spaces $X$ admitting a $P$-base, i.e., an indexed family $(U_\\alpha)_{\\alpha\\in P}$ of subsets of $X\\times X$ such that $U_\\beta\\subset U_\\alpha$ for all $\\alpha\\le\\beta$ in $P$ and for every $x\\in X$ the family $(U_\\alpha[x])_{\\alpha\\in P}$ of balls $U_\\alpha[x]=\\{y\\in X:(x,y)\\in U_\\alpha\\}$ is a neighborhood base at $x$. A $P$-base $(U_\\alpha)_{\\alpha\\in P}$ for $X$ is called locally uniform if the family of entourages $(U_\\alpha U_\\alpha^{-1}U_\\alpha)_{\\alpha\\in P}$ remains a $P$-base for $X$. A topological space is first-countable if and only if it has an $\\omega$-base. By Moore's Metrization Theorem, a topological space is metrizable if and only if it is a $T_0$-space with a locally uniform $\\omega$-base. \nIn the paper we shall study topological spaces possessing a (locally uniform) $\\omega^\\omega$-base. Our results show that spaces with an $\\omega^\\omega$-base share some common properties with first countable spaces, in particular, many known upper bounds on the cardinality of first-countable spaces remain true for countably tight $\\omega^\\omega$-based topological spaces. On the other hand, topological spaces with a locally uniform $\\omega^\\omega$-base have many properties, typical for generalized metric spaces. Also we study Tychonoff spaces whose universal (pre- or quasi-) uniformity has an $\\omega^\\omega$-base and show that such spaces are close to being $\\sigma$-compact.","PeriodicalId":51016,"journal":{"name":"Dissertationes Mathematicae","volume":"1 1","pages":""},"PeriodicalIF":1.5000,"publicationDate":"2016-07-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"7","resultStr":"{\"title\":\"Topological spaces with an $\\\\omega^{\\\\omega}$-base\",\"authors\":\"T. Banakh\",\"doi\":\"10.4064/dm762-4-2018\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"Given a partially ordered set $P$ we study properties of topological spaces $X$ admitting a $P$-base, i.e., an indexed family $(U_\\\\alpha)_{\\\\alpha\\\\in P}$ of subsets of $X\\\\times X$ such that $U_\\\\beta\\\\subset U_\\\\alpha$ for all $\\\\alpha\\\\le\\\\beta$ in $P$ and for every $x\\\\in X$ the family $(U_\\\\alpha[x])_{\\\\alpha\\\\in P}$ of balls $U_\\\\alpha[x]=\\\\{y\\\\in X:(x,y)\\\\in U_\\\\alpha\\\\}$ is a neighborhood base at $x$. A $P$-base $(U_\\\\alpha)_{\\\\alpha\\\\in P}$ for $X$ is called locally uniform if the family of entourages $(U_\\\\alpha U_\\\\alpha^{-1}U_\\\\alpha)_{\\\\alpha\\\\in P}$ remains a $P$-base for $X$. A topological space is first-countable if and only if it has an $\\\\omega$-base. By Moore's Metrization Theorem, a topological space is metrizable if and only if it is a $T_0$-space with a locally uniform $\\\\omega$-base. \\nIn the paper we shall study topological spaces possessing a (locally uniform) $\\\\omega^\\\\omega$-base. Our results show that spaces with an $\\\\omega^\\\\omega$-base share some common properties with first countable spaces, in particular, many known upper bounds on the cardinality of first-countable spaces remain true for countably tight $\\\\omega^\\\\omega$-based topological spaces. On the other hand, topological spaces with a locally uniform $\\\\omega^\\\\omega$-base have many properties, typical for generalized metric spaces. Also we study Tychonoff spaces whose universal (pre- or quasi-) uniformity has an $\\\\omega^\\\\omega$-base and show that such spaces are close to being $\\\\sigma$-compact.\",\"PeriodicalId\":51016,\"journal\":{\"name\":\"Dissertationes Mathematicae\",\"volume\":\"1 1\",\"pages\":\"\"},\"PeriodicalIF\":1.5000,\"publicationDate\":\"2016-07-27\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"\",\"citationCount\":\"7\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"Dissertationes Mathematicae\",\"FirstCategoryId\":\"100\",\"ListUrlMain\":\"https://doi.org/10.4064/dm762-4-2018\",\"RegionNum\":3,\"RegionCategory\":\"数学\",\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"Q1\",\"JCRName\":\"MATHEMATICS\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"Dissertationes Mathematicae","FirstCategoryId":"100","ListUrlMain":"https://doi.org/10.4064/dm762-4-2018","RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q1","JCRName":"MATHEMATICS","Score":null,"Total":0}
Given a partially ordered set $P$ we study properties of topological spaces $X$ admitting a $P$-base, i.e., an indexed family $(U_\alpha)_{\alpha\in P}$ of subsets of $X\times X$ such that $U_\beta\subset U_\alpha$ for all $\alpha\le\beta$ in $P$ and for every $x\in X$ the family $(U_\alpha[x])_{\alpha\in P}$ of balls $U_\alpha[x]=\{y\in X:(x,y)\in U_\alpha\}$ is a neighborhood base at $x$. A $P$-base $(U_\alpha)_{\alpha\in P}$ for $X$ is called locally uniform if the family of entourages $(U_\alpha U_\alpha^{-1}U_\alpha)_{\alpha\in P}$ remains a $P$-base for $X$. A topological space is first-countable if and only if it has an $\omega$-base. By Moore's Metrization Theorem, a topological space is metrizable if and only if it is a $T_0$-space with a locally uniform $\omega$-base.
In the paper we shall study topological spaces possessing a (locally uniform) $\omega^\omega$-base. Our results show that spaces with an $\omega^\omega$-base share some common properties with first countable spaces, in particular, many known upper bounds on the cardinality of first-countable spaces remain true for countably tight $\omega^\omega$-based topological spaces. On the other hand, topological spaces with a locally uniform $\omega^\omega$-base have many properties, typical for generalized metric spaces. Also we study Tychonoff spaces whose universal (pre- or quasi-) uniformity has an $\omega^\omega$-base and show that such spaces are close to being $\sigma$-compact.
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