{"title":"属于D空间的Box和nabla产品","authors":"H.A. Barriga-Acosta , P.M. Gartside","doi":"10.1016/j.indag.2023.04.002","DOIUrl":null,"url":null,"abstract":"<div><p>A space <span><math><mi>X</mi></math></span> is <span><math><mi>D</mi></math></span> if for every assignment, <span><math><mi>U</mi></math></span><span>, of an open neighborhood to each point </span><span><math><mi>x</mi></math></span> in <span><math><mi>X</mi></math></span> there is a closed discrete <span><math><mi>D</mi></math></span> such that <span><math><mrow><mo>⋃</mo><mrow><mo>{</mo><mi>U</mi><mrow><mo>(</mo><mi>x</mi><mo>)</mo></mrow><mo>:</mo><mi>x</mi><mo>∈</mo><mi>D</mi><mo>}</mo></mrow><mo>=</mo><mi>X</mi></mrow></math></span>. The box product, <span><math><mrow><mo>□</mo><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span>, is <span><math><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></math></span> with topology generated by all <span><math><mrow><msub><mrow><mo>∏</mo></mrow><mrow><mi>n</mi></mrow></msub><msub><mrow><mi>U</mi></mrow><mrow><mi>n</mi></mrow></msub></mrow></math></span>, where every <span><math><msub><mrow><mi>U</mi></mrow><mrow><mi>n</mi></mrow></msub></math></span> is open. The nabla product, <span><math><mrow><mo>∇</mo><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span>, is obtained from <span><math><mrow><mo>□</mo><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span> by quotienting out mod-finite. The weight of <span><math><mi>X</mi></math></span>, <span><math><mrow><mi>w</mi><mrow><mo>(</mo><mi>X</mi><mo>)</mo></mrow></mrow></math></span>, is the minimal size of a base, while <span><math><mrow><mi>d</mi><mo>=</mo><mo>cof</mo><msup><mrow><mi>ω</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span>.</p><p>It is shown that there are specific compact spaces <span><math><mi>X</mi></math></span> such that <span><math><mrow><mo>□</mo><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span> and <span><math><mrow><mo>∇</mo><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span> are not <span><math><mi>D</mi></math></span>, but in general:</p><p>(1) <span><math><mrow><mo>□</mo><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span> and <span><math><mrow><mo>∇</mo><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span> are hereditarily <span><math><mi>D</mi></math></span> if <span><math><mi>X</mi></math></span> is scattered and either hereditarily paracompact or of finite scattered height, or if <span><math><mi>X</mi></math></span> is metrizable (and <span><math><mrow><mi>w</mi><mrow><mo>(</mo><mi>X</mi><mo>)</mo></mrow><mo>≤</mo><mi>d</mi></mrow></math></span> for <span><math><mrow><mo>□</mo><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span>);</p><p>(2) <span><math><mrow><mo>∇</mo><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span> is hereditarily <span><math><mi>D</mi></math></span> if <span><math><mi>X</mi></math></span> is first countable and <span><math><mrow><mi>w</mi><mrow><mo>(</mo><mi>X</mi><mo>)</mo></mrow><mo>≤</mo><msub><mrow><mi>ω</mi></mrow><mrow><mn>1</mn></mrow></msub></mrow></math></span>, or <em>consistently</em> if <span><math><mi>X</mi></math></span> is first countable and <span><math><mrow><mo>|</mo><mi>X</mi><mo>|</mo><mo>≤</mo><mi>c</mi></mrow></math></span>, or <span><math><mrow><mi>w</mi><mrow><mo>(</mo><mi>X</mi><mo>)</mo></mrow><mo>≤</mo><msub><mrow><mi>ω</mi></mrow><mrow><mn>1</mn></mrow></msub></mrow></math></span>; and</p><p>(3) <span><math><mrow><mo>□</mo><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span> is <span><math><mi>D</mi></math></span> <em>consistently</em> if <span><math><mi>X</mi></math></span> is compact and either first countable or <span><math><mrow><mi>w</mi><mrow><mo>(</mo><mi>X</mi><mo>)</mo></mrow><mo>≤</mo><msub><mrow><mi>ω</mi></mrow><mrow><mn>1</mn></mrow></msub></mrow></math></span>.</p></div>","PeriodicalId":0,"journal":{"name":"","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-05-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Box and nabla products that are D-spaces\",\"authors\":\"H.A. Barriga-Acosta , P.M. Gartside\",\"doi\":\"10.1016/j.indag.2023.04.002\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"<div><p>A space <span><math><mi>X</mi></math></span> is <span><math><mi>D</mi></math></span> if for every assignment, <span><math><mi>U</mi></math></span><span>, of an open neighborhood to each point </span><span><math><mi>x</mi></math></span> in <span><math><mi>X</mi></math></span> there is a closed discrete <span><math><mi>D</mi></math></span> such that <span><math><mrow><mo>⋃</mo><mrow><mo>{</mo><mi>U</mi><mrow><mo>(</mo><mi>x</mi><mo>)</mo></mrow><mo>:</mo><mi>x</mi><mo>∈</mo><mi>D</mi><mo>}</mo></mrow><mo>=</mo><mi>X</mi></mrow></math></span>. The box product, <span><math><mrow><mo>□</mo><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span>, is <span><math><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></math></span> with topology generated by all <span><math><mrow><msub><mrow><mo>∏</mo></mrow><mrow><mi>n</mi></mrow></msub><msub><mrow><mi>U</mi></mrow><mrow><mi>n</mi></mrow></msub></mrow></math></span>, where every <span><math><msub><mrow><mi>U</mi></mrow><mrow><mi>n</mi></mrow></msub></math></span> is open. The nabla product, <span><math><mrow><mo>∇</mo><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span>, is obtained from <span><math><mrow><mo>□</mo><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span> by quotienting out mod-finite. The weight of <span><math><mi>X</mi></math></span>, <span><math><mrow><mi>w</mi><mrow><mo>(</mo><mi>X</mi><mo>)</mo></mrow></mrow></math></span>, is the minimal size of a base, while <span><math><mrow><mi>d</mi><mo>=</mo><mo>cof</mo><msup><mrow><mi>ω</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span>.</p><p>It is shown that there are specific compact spaces <span><math><mi>X</mi></math></span> such that <span><math><mrow><mo>□</mo><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span> and <span><math><mrow><mo>∇</mo><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span> are not <span><math><mi>D</mi></math></span>, but in general:</p><p>(1) <span><math><mrow><mo>□</mo><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span> and <span><math><mrow><mo>∇</mo><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span> are hereditarily <span><math><mi>D</mi></math></span> if <span><math><mi>X</mi></math></span> is scattered and either hereditarily paracompact or of finite scattered height, or if <span><math><mi>X</mi></math></span> is metrizable (and <span><math><mrow><mi>w</mi><mrow><mo>(</mo><mi>X</mi><mo>)</mo></mrow><mo>≤</mo><mi>d</mi></mrow></math></span> for <span><math><mrow><mo>□</mo><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span>);</p><p>(2) <span><math><mrow><mo>∇</mo><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span> is hereditarily <span><math><mi>D</mi></math></span> if <span><math><mi>X</mi></math></span> is first countable and <span><math><mrow><mi>w</mi><mrow><mo>(</mo><mi>X</mi><mo>)</mo></mrow><mo>≤</mo><msub><mrow><mi>ω</mi></mrow><mrow><mn>1</mn></mrow></msub></mrow></math></span>, or <em>consistently</em> if <span><math><mi>X</mi></math></span> is first countable and <span><math><mrow><mo>|</mo><mi>X</mi><mo>|</mo><mo>≤</mo><mi>c</mi></mrow></math></span>, or <span><math><mrow><mi>w</mi><mrow><mo>(</mo><mi>X</mi><mo>)</mo></mrow><mo>≤</mo><msub><mrow><mi>ω</mi></mrow><mrow><mn>1</mn></mrow></msub></mrow></math></span>; and</p><p>(3) <span><math><mrow><mo>□</mo><msup><mrow><mi>X</mi></mrow><mrow><mi>ω</mi></mrow></msup></mrow></math></span> is <span><math><mi>D</mi></math></span> <em>consistently</em> if <span><math><mi>X</mi></math></span> is compact and either first countable or <span><math><mrow><mi>w</mi><mrow><mo>(</mo><mi>X</mi><mo>)</mo></mrow><mo>≤</mo><msub><mrow><mi>ω</mi></mrow><mrow><mn>1</mn></mrow></msub></mrow></math></span>.</p></div>\",\"PeriodicalId\":0,\"journal\":{\"name\":\"\",\"volume\":null,\"pages\":null},\"PeriodicalIF\":0.0,\"publicationDate\":\"2023-05-03\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"\",\"FirstCategoryId\":\"100\",\"ListUrlMain\":\"https://www.sciencedirect.com/science/article/pii/S0019357723000381\",\"RegionNum\":0,\"RegionCategory\":null,\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"\",\"JCRName\":\"\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"","FirstCategoryId":"100","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S0019357723000381","RegionNum":0,"RegionCategory":null,"ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"","JCRName":"","Score":null,"Total":0}
A space is if for every assignment, , of an open neighborhood to each point in there is a closed discrete such that . The box product, , is with topology generated by all , where every is open. The nabla product, , is obtained from by quotienting out mod-finite. The weight of , , is the minimal size of a base, while .
It is shown that there are specific compact spaces such that and are not , but in general:
(1) and are hereditarily if is scattered and either hereditarily paracompact or of finite scattered height, or if is metrizable (and for );
(2) is hereditarily if is first countable and , or consistently if is first countable and , or ; and
(3) is consistently if is compact and either first countable or .