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class="vector-pinned-container"> <div id="vector-toc" class="vector-toc vector-pinnable-element"> <div class="vector-pinnable-header vector-toc-pinnable-header vector-pinnable-header-pinned" data-feature-name="toc-pinned" data-pinnable-element-id="vector-toc" > <h2 class="vector-pinnable-header-label">目录</h2> <button class="vector-pinnable-header-toggle-button vector-pinnable-header-pin-button" data-event-name="pinnable-header.vector-toc.pin">移至侧栏</button> <button class="vector-pinnable-header-toggle-button vector-pinnable-header-unpin-button" data-event-name="pinnable-header.vector-toc.unpin">隐藏</button> </div> <ul class="vector-toc-contents" id="mw-panel-toc-list"> <li id="toc-mw-content-text" class="vector-toc-list-item vector-toc-level-1"> <a href="#" class="vector-toc-link"> <div class="vector-toc-text">序言</div> </a> </li> <li id="toc-数学性质" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#数学性质"> <div class="vector-toc-text"> <span class="vector-toc-numb">1</span> <span>数学性质</span> </div> </a> <button aria-controls="toc-数学性质-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>开关数学性质子章节</span> </button> <ul id="toc-数学性质-sublist" class="vector-toc-list"> <li id="toc-基本运算" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#基本运算"> <div class="vector-toc-text"> <span class="vector-toc-numb">1.1</span> <span>基本运算</span> </div> </a> <ul id="toc-基本运算-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-在科学中" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#在科学中"> <div class="vector-toc-text"> <span class="vector-toc-numb">2</span> <span>在科学中</span> </div> </a> <ul id="toc-在科学中-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-在人类文化中" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#在人类文化中"> <div class="vector-toc-text"> <span class="vector-toc-numb">3</span> <span>在人类文化中</span> </div> </a> <ul id="toc-在人类文化中-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-在其他領域中" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#在其他領域中"> <div class="vector-toc-text"> <span class="vector-toc-numb">4</span> <span>在其他領域中</span> </div> </a> <ul id="toc-在其他領域中-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-参考文献" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#参考文献"> <div class="vector-toc-text"> <span class="vector-toc-numb">5</span> <span>参考文献</span> </div> </a> <ul id="toc-参考文献-sublist" class="vector-toc-list"> </ul> </li> </ul> </div> </div> </nav> </div> </div> <div class="mw-content-container"> <main id="content" class="mw-body"> <header class="mw-body-header vector-page-titlebar"> <nav aria-label="目录" class="vector-toc-landmark"> <div id="vector-page-titlebar-toc" class="vector-dropdown vector-page-titlebar-toc vector-button-flush-left" title="目录" > <input type="checkbox" id="vector-page-titlebar-toc-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-vector-page-titlebar-toc" class="vector-dropdown-checkbox " aria-label="开关目录" > <label id="vector-page-titlebar-toc-label" for="vector-page-titlebar-toc-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--icon-only " aria-hidden="true" ><span class="vector-icon mw-ui-icon-listBullet mw-ui-icon-wikimedia-listBullet"></span> <span class="vector-dropdown-label-text">开关目录</span> </label> <div class="vector-dropdown-content"> <div id="vector-page-titlebar-toc-unpinned-container" class="vector-unpinned-container"> </div> </div> </div> </nav> <h1 id="firstHeading" class="firstHeading mw-first-heading"><span class="mw-page-title-main">21</span></h1> <div id="p-lang-btn" class="vector-dropdown mw-portlet mw-portlet-lang" > <input type="checkbox" id="p-lang-btn-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-p-lang-btn" class="vector-dropdown-checkbox mw-interlanguage-selector" aria-label="前往另一种语言写成的文章。98种语言可用" > <label id="p-lang-btn-label" for="p-lang-btn-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--action-progressive mw-portlet-lang-heading-98" aria-hidden="true" ><span class="vector-icon mw-ui-icon-language-progressive mw-ui-icon-wikimedia-language-progressive"></span> <span class="vector-dropdown-label-text">98种语言</span> </label> <div class="vector-dropdown-content"> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li class="interlanguage-link interwiki-ab mw-list-item"><a href="https://ab.wikipedia.org/wiki/21_(%D0%B0%D1%85%D1%8B%D4%A5%D1%85%D1%8C%D0%B0%D3%A1%D0%B0%D1%80%D0%B0)" title="21 (ахыԥхьаӡара) – 阿布哈西亚语" lang="ab" hreflang="ab" data-title="21 (ахыԥхьаӡара)" data-language-autonym="Аԥсшәа" data-language-local-name="阿布哈西亚语" class="interlanguage-link-target"><span>Аԥсшәа</span></a></li><li class="interlanguage-link interwiki-ar mw-list-item"><a href="https://ar.wikipedia.org/wiki/21_(%D8%B9%D8%AF%D8%AF)" title="21 (عدد) – 阿拉伯语" lang="ar" hreflang="ar" data-title="21 (عدد)" data-language-autonym="العربية" data-language-local-name="阿拉伯语" class="interlanguage-link-target"><span>العربية</span></a></li><li class="interlanguage-link interwiki-ary mw-list-item"><a href="https://ary.wikipedia.org/wiki/21" title="21 – 摩洛哥阿拉伯文" lang="ary" hreflang="ary" data-title="21" data-language-autonym="الدارجة" data-language-local-name="摩洛哥阿拉伯文" class="interlanguage-link-target"><span>الدارجة</span></a></li><li class="interlanguage-link interwiki-az mw-list-item"><a href="https://az.wikipedia.org/wiki/21_(%C9%99d%C9%99d)" title="21 (ədəd) – 阿塞拜疆语" lang="az" hreflang="az" data-title="21 (ədəd)" data-language-autonym="Azərbaycanca" data-language-local-name="阿塞拜疆语" class="interlanguage-link-target"><span>Azərbaycanca</span></a></li><li class="interlanguage-link interwiki-azb mw-list-item"><a href="https://azb.wikipedia.org/wiki/%DB%B2%DB%B1_(%D8%B3%D8%A7%DB%8C%DB%8C)" title="۲۱ (سایی) – South Azerbaijani" lang="azb" hreflang="azb" data-title="۲۱ (سایی)" data-language-autonym="تۆرکجه" data-language-local-name="South Azerbaijani" class="interlanguage-link-target"><span>تۆرکجه</span></a></li><li class="interlanguage-link interwiki-ban mw-list-item"><a href="https://ban.wikipedia.org/wiki/21_(angka)" title="21 (angka) – 巴厘语" lang="ban" hreflang="ban" data-title="21 (angka)" data-language-autonym="Basa Bali" data-language-local-name="巴厘语" class="interlanguage-link-target"><span>Basa Bali</span></a></li><li class="interlanguage-link interwiki-bg mw-list-item"><a href="https://bg.wikipedia.org/wiki/21_(%D1%87%D0%B8%D1%81%D0%BB%D0%BE)" title="21 (число) – 保加利亚语" lang="bg" hreflang="bg" data-title="21 (число)" data-language-autonym="Български" data-language-local-name="保加利亚语" class="interlanguage-link-target"><span>Български</span></a></li><li class="interlanguage-link interwiki-bo mw-list-item"><a href="https://bo.wikipedia.org/wiki/%E0%BC%A2%E0%BC%A1_(%E0%BD%82%E0%BE%B2%E0%BD%84%E0%BD%A6%E0%BC%8B%E0%BD%80%E0%BC%8D)" title="༢༡ (གྲངས་ཀ།) – 藏语" lang="bo" hreflang="bo" data-title="༢༡ (གྲངས་ཀ།)" data-language-autonym="བོད་ཡིག" data-language-local-name="藏语" class="interlanguage-link-target"><span>བོད་ཡིག</span></a></li><li class="interlanguage-link interwiki-ca mw-list-item"><a href="https://ca.wikipedia.org/wiki/Vint-i-u" title="Vint-i-u – 加泰罗尼亚语" lang="ca" hreflang="ca" data-title="Vint-i-u" data-language-autonym="Català" data-language-local-name="加泰罗尼亚语" class="interlanguage-link-target"><span>Català</span></a></li><li class="interlanguage-link interwiki-cdo mw-list-item"><a href="https://cdo.wikipedia.org/wiki/21" title="21 – Mindong" lang="cdo" hreflang="cdo" data-title="21" data-language-autonym="閩東語 / Mìng-dĕ̤ng-ngṳ̄" data-language-local-name="Mindong" class="interlanguage-link-target"><span>閩東語 / Mìng-dĕ̤ng-ngṳ̄</span></a></li><li class="interlanguage-link interwiki-ckb mw-list-item"><a href="https://ckb.wikipedia.org/wiki/%D9%A2%D9%A1_(%DA%98%D9%85%D8%A7%D8%B1%DB%95)" title="٢١ (ژمارە) – 中库尔德语" lang="ckb" hreflang="ckb" data-title="٢١ (ژمارە)" data-language-autonym="کوردی" data-language-local-name="中库尔德语" class="interlanguage-link-target"><span>کوردی</span></a></li><li class="interlanguage-link interwiki-cs mw-list-item"><a href="https://cs.wikipedia.org/wiki/21_(%C4%8D%C3%ADslo)" title="21 (číslo) – 捷克语" lang="cs" hreflang="cs" data-title="21 (číslo)" data-language-autonym="Čeština" data-language-local-name="捷克语" class="interlanguage-link-target"><span>Čeština</span></a></li><li class="interlanguage-link interwiki-cv mw-list-item"><a href="https://cv.wikipedia.org/wiki/21_(%D1%85%D0%B8%D1%81%D0%B5%D0%BF)" title="21 (хисеп) – 楚瓦什语" lang="cv" hreflang="cv" data-title="21 (хисеп)" data-language-autonym="Чӑвашла" data-language-local-name="楚瓦什语" class="interlanguage-link-target"><span>Чӑвашла</span></a></li><li class="interlanguage-link interwiki-cy mw-list-item"><a href="https://cy.wikipedia.org/wiki/21_(rhif)" title="21 (rhif) – 威尔士语" lang="cy" hreflang="cy" data-title="21 (rhif)" data-language-autonym="Cymraeg" data-language-local-name="威尔士语" class="interlanguage-link-target"><span>Cymraeg</span></a></li><li class="interlanguage-link interwiki-da mw-list-item"><a href="https://da.wikipedia.org/wiki/21_(tal)" title="21 (tal) – 丹麦语" lang="da" hreflang="da" data-title="21 (tal)" data-language-autonym="Dansk" data-language-local-name="丹麦语" class="interlanguage-link-target"><span>Dansk</span></a></li><li class="interlanguage-link interwiki-de mw-list-item"><a href="https://de.wikipedia.org/wiki/Einundzwanzig" title="Einundzwanzig – 德语" lang="de" hreflang="de" data-title="Einundzwanzig" data-language-autonym="Deutsch" data-language-local-name="德语" class="interlanguage-link-target"><span>Deutsch</span></a></li><li class="interlanguage-link interwiki-dz mw-list-item"><a href="https://dz.wikipedia.org/wiki/%E0%BC%A2%E0%BC%A1" title="༢༡ – 宗卡语" lang="dz" hreflang="dz" data-title="༢༡" data-language-autonym="ཇོང་ཁ" data-language-local-name="宗卡语" class="interlanguage-link-target"><span>ཇོང་ཁ</span></a></li><li class="interlanguage-link interwiki-el mw-list-item"><a href="https://el.wikipedia.org/wiki/21_(%CE%B1%CF%81%CE%B9%CE%B8%CE%BC%CF%8C%CF%82)" title="21 (αριθμός) – 希腊语" lang="el" hreflang="el" data-title="21 (αριθμός)" data-language-autonym="Ελληνικά" data-language-local-name="希腊语" class="interlanguage-link-target"><span>Ελληνικά</span></a></li><li class="interlanguage-link interwiki-eml mw-list-item"><a href="https://eml.wikipedia.org/wiki/21_(n%C3%B9mer)" title="21 (nùmer) – Emiliano-Romagnolo" lang="egl" hreflang="egl" data-title="21 (nùmer)" data-language-autonym="Emiliàn e rumagnòl" data-language-local-name="Emiliano-Romagnolo" class="interlanguage-link-target"><span>Emiliàn e rumagnòl</span></a></li><li class="interlanguage-link interwiki-en mw-list-item"><a href="https://en.wikipedia.org/wiki/21_(number)" title="21 (number) – 英语" lang="en" hreflang="en" data-title="21 (number)" data-language-autonym="English" data-language-local-name="英语" class="interlanguage-link-target"><span>English</span></a></li><li class="interlanguage-link interwiki-eo mw-list-item"><a href="https://eo.wikipedia.org/wiki/21_(nombro)" title="21 (nombro) – 世界语" lang="eo" hreflang="eo" data-title="21 (nombro)" data-language-autonym="Esperanto" data-language-local-name="世界语" class="interlanguage-link-target"><span>Esperanto</span></a></li><li class="interlanguage-link interwiki-es mw-list-item"><a href="https://es.wikipedia.org/wiki/Veintiuno" title="Veintiuno – 西班牙语" lang="es" hreflang="es" data-title="Veintiuno" data-language-autonym="Español" data-language-local-name="西班牙语" class="interlanguage-link-target"><span>Español</span></a></li><li class="interlanguage-link interwiki-eu mw-list-item"><a href="https://eu.wikipedia.org/wiki/Hogeita_bat" title="Hogeita bat – 巴斯克语" lang="eu" hreflang="eu" data-title="Hogeita bat" data-language-autonym="Euskara" data-language-local-name="巴斯克语" class="interlanguage-link-target"><span>Euskara</span></a></li><li class="interlanguage-link interwiki-fa mw-list-item"><a href="https://fa.wikipedia.org/wiki/%DB%B2%DB%B1_(%D8%B9%D8%AF%D8%AF)" title="۲۱ (عدد) – 波斯语" lang="fa" hreflang="fa" data-title="۲۱ (عدد)" data-language-autonym="فارسی" data-language-local-name="波斯语" class="interlanguage-link-target"><span>فارسی</span></a></li><li class="interlanguage-link interwiki-ff mw-list-item"><a href="https://ff.wikipedia.org/wiki/Noogas_e_go%E2%80%99o" title="Noogas e go’o – 富拉语" lang="ff" hreflang="ff" data-title="Noogas e go’o" data-language-autonym="Fulfulde" data-language-local-name="富拉语" class="interlanguage-link-target"><span>Fulfulde</span></a></li><li class="interlanguage-link interwiki-fi mw-list-item"><a href="https://fi.wikipedia.org/wiki/21_(luku)" title="21 (luku) – 芬兰语" lang="fi" hreflang="fi" data-title="21 (luku)" data-language-autonym="Suomi" data-language-local-name="芬兰语" class="interlanguage-link-target"><span>Suomi</span></a></li><li class="interlanguage-link interwiki-fj mw-list-item"><a href="https://fj.wikipedia.org/wiki/Rua_ka_dua" title="Rua ka dua – 斐济语" lang="fj" hreflang="fj" data-title="Rua ka dua" data-language-autonym="Na Vosa Vakaviti" data-language-local-name="斐济语" class="interlanguage-link-target"><span>Na Vosa Vakaviti</span></a></li><li class="interlanguage-link interwiki-fo mw-list-item"><a href="https://fo.wikipedia.org/wiki/21_(tal)" title="21 (tal) – 法罗语" lang="fo" hreflang="fo" data-title="21 (tal)" data-language-autonym="Føroyskt" data-language-local-name="法罗语" class="interlanguage-link-target"><span>Føroyskt</span></a></li><li class="interlanguage-link interwiki-fr mw-list-item"><a href="https://fr.wikipedia.org/wiki/21_(nombre)" title="21 (nombre) – 法语" lang="fr" hreflang="fr" data-title="21 (nombre)" data-language-autonym="Français" data-language-local-name="法语" class="interlanguage-link-target"><span>Français</span></a></li><li class="interlanguage-link interwiki-ga mw-list-item"><a href="https://ga.wikipedia.org/wiki/21_(uimhir)" title="21 (uimhir) – 爱尔兰语" lang="ga" hreflang="ga" data-title="21 (uimhir)" data-language-autonym="Gaeilge" data-language-local-name="爱尔兰语" class="interlanguage-link-target"><span>Gaeilge</span></a></li><li class="interlanguage-link interwiki-gan mw-list-item"><a href="https://gan.wikipedia.org/wiki/21" title="21 – 赣语" lang="gan" hreflang="gan" data-title="21" data-language-autonym="贛語" data-language-local-name="赣语" class="interlanguage-link-target"><span>贛語</span></a></li><li class="interlanguage-link interwiki-ha mw-list-item"><a href="https://ha.wikipedia.org/wiki/21_(al%C6%99alami)" title="21 (alƙalami) – 豪萨语" lang="ha" hreflang="ha" data-title="21 (alƙalami)" data-language-autonym="Hausa" data-language-local-name="豪萨语" class="interlanguage-link-target"><span>Hausa</span></a></li><li class="interlanguage-link interwiki-he mw-list-item"><a href="https://he.wikipedia.org/wiki/21_(%D7%9E%D7%A1%D7%A4%D7%A8)" title="21 (מספר) – 希伯来语" lang="he" hreflang="he" data-title="21 (מספר)" data-language-autonym="עברית" data-language-local-name="希伯来语" class="interlanguage-link-target"><span>עברית</span></a></li><li class="interlanguage-link interwiki-hi mw-list-item"><a href="https://hi.wikipedia.org/wiki/%E0%A5%A8%E0%A5%A7_(%E0%A4%B8%E0%A4%82%E0%A4%96%E0%A5%8D%E0%A4%AF%E0%A4%BE)" title="२१ (संख्या) – 印地语" lang="hi" hreflang="hi" data-title="२१ (संख्या)" data-language-autonym="हिन्दी" data-language-local-name="印地语" class="interlanguage-link-target"><span>हिन्दी</span></a></li><li class="interlanguage-link interwiki-ht mw-list-item"><a href="https://ht.wikipedia.org/wiki/21_(nonm)" title="21 (nonm) – 海地克里奥尔语" lang="ht" hreflang="ht" data-title="21 (nonm)" data-language-autonym="Kreyòl ayisyen" data-language-local-name="海地克里奥尔语" class="interlanguage-link-target"><span>Kreyòl ayisyen</span></a></li><li class="interlanguage-link interwiki-hu mw-list-item"><a href="https://hu.wikipedia.org/wiki/21_(sz%C3%A1m)" title="21 (szám) – 匈牙利语" lang="hu" hreflang="hu" data-title="21 (szám)" data-language-autonym="Magyar" data-language-local-name="匈牙利语" class="interlanguage-link-target"><span>Magyar</span></a></li><li class="interlanguage-link interwiki-hy mw-list-item"><a href="https://hy.wikipedia.org/wiki/21_(%D5%A9%D5%AB%D5%BE)" title="21 (թիվ) – 亚美尼亚语" lang="hy" hreflang="hy" data-title="21 (թիվ)" data-language-autonym="Հայերեն" data-language-local-name="亚美尼亚语" class="interlanguage-link-target"><span>Հայերեն</span></a></li><li class="interlanguage-link interwiki-ia mw-list-item"><a href="https://ia.wikipedia.org/wiki/21_(numero)" title="21 (numero) – 国际语" lang="ia" hreflang="ia" data-title="21 (numero)" data-language-autonym="Interlingua" data-language-local-name="国际语" class="interlanguage-link-target"><span>Interlingua</span></a></li><li class="interlanguage-link interwiki-id mw-list-item"><a href="https://id.wikipedia.org/wiki/21_(angka)" title="21 (angka) – 印度尼西亚语" lang="id" hreflang="id" data-title="21 (angka)" data-language-autonym="Bahasa Indonesia" data-language-local-name="印度尼西亚语" class="interlanguage-link-target"><span>Bahasa Indonesia</span></a></li><li class="interlanguage-link interwiki-ig mw-list-item"><a href="https://ig.wikipedia.org/wiki/Iri_ab%E1%BB%A5o_na_otu" title="Iri abụo na otu – 伊博语" lang="ig" hreflang="ig" data-title="Iri abụo na otu" data-language-autonym="Igbo" data-language-local-name="伊博语" class="interlanguage-link-target"><span>Igbo</span></a></li><li class="interlanguage-link interwiki-ik mw-list-item"><a href="https://ik.wikipedia.org/wiki/I%C3%B1ui%C3%B1aq_atausiq" title="Iñuiñaq atausiq – 伊努皮克语" lang="ik" hreflang="ik" data-title="Iñuiñaq atausiq" data-language-autonym="Iñupiatun" data-language-local-name="伊努皮克语" class="interlanguage-link-target"><span>Iñupiatun</span></a></li><li class="interlanguage-link interwiki-it mw-list-item"><a href="https://it.wikipedia.org/wiki/21_(numero)" title="21 (numero) – 意大利语" lang="it" hreflang="it" data-title="21 (numero)" data-language-autonym="Italiano" data-language-local-name="意大利语" class="interlanguage-link-target"><span>Italiano</span></a></li><li class="interlanguage-link interwiki-ja mw-list-item"><a href="https://ja.wikipedia.org/wiki/21" title="21 – 日语" lang="ja" hreflang="ja" data-title="21" data-language-autonym="日本語" data-language-local-name="日语" class="interlanguage-link-target"><span>日本語</span></a></li><li class="interlanguage-link interwiki-ka mw-list-item"><a href="https://ka.wikipedia.org/wiki/21_(%E1%83%A0%E1%83%98%E1%83%AA%E1%83%AE%E1%83%95%E1%83%98)" title="21 (რიცხვი) – 格鲁吉亚语" lang="ka" hreflang="ka" data-title="21 (რიცხვი)" data-language-autonym="ქართული" data-language-local-name="格鲁吉亚语" class="interlanguage-link-target"><span>ქართული</span></a></li><li class="interlanguage-link interwiki-kcg mw-list-item"><a href="https://kcg.wikipedia.org/wiki/21_(la%CC%B1mba)" title="21 (la̱mba) – 卡塔布语" lang="kcg" hreflang="kcg" data-title="21 (la̱mba)" data-language-autonym="Tyap" data-language-local-name="卡塔布语" class="interlanguage-link-target"><span>Tyap</span></a></li><li class="interlanguage-link interwiki-ko mw-list-item"><a href="https://ko.wikipedia.org/wiki/21" title="21 – 韩语" lang="ko" hreflang="ko" data-title="21" data-language-autonym="한국어" data-language-local-name="韩语" class="interlanguage-link-target"><span>한국어</span></a></li><li class="interlanguage-link interwiki-ks mw-list-item"><a href="https://ks.wikipedia.org/wiki/21_(%DA%AF%D8%B1%D9%8E%D9%9B%D9%86%D9%9B%D8%AF)" title="21 (گرَٛنٛد) – 克什米尔语" lang="ks" hreflang="ks" data-title="21 (گرَٛنٛد)" data-language-autonym="कॉशुर / کٲشُر" data-language-local-name="克什米尔语" class="interlanguage-link-target"><span>कॉशुर / کٲشُر</span></a></li><li class="interlanguage-link interwiki-la mw-list-item"><a href="https://la.wikipedia.org/wiki/Viginti_unus" title="Viginti unus – 拉丁语" lang="la" hreflang="la" data-title="Viginti unus" data-language-autonym="Latina" data-language-local-name="拉丁语" class="interlanguage-link-target"><span>Latina</span></a></li><li class="interlanguage-link interwiki-lbe mw-list-item"><a href="https://lbe.wikipedia.org/wiki/%D0%9A%D1%8C%D1%83%D0%BD%D0%B8%D1%8F_%D1%86%D0%B0%D0%B2%D0%B0" title="Кьуния цава – Lak" lang="lbe" hreflang="lbe" data-title="Кьуния цава" data-language-autonym="Лакку" data-language-local-name="Lak" class="interlanguage-link-target"><span>Лакку</span></a></li><li class="interlanguage-link interwiki-lg mw-list-item"><a href="https://lg.wikipedia.org/wiki/Abiri_mu_emu" title="Abiri mu emu – 卢干达语" lang="lg" hreflang="lg" data-title="Abiri mu emu" data-language-autonym="Luganda" data-language-local-name="卢干达语" class="interlanguage-link-target"><span>Luganda</span></a></li><li class="interlanguage-link interwiki-ln mw-list-item"><a href="https://ln.wikipedia.org/wiki/21_(motuya)" title="21 (motuya) – 林加拉语" lang="ln" hreflang="ln" data-title="21 (motuya)" data-language-autonym="Lingála" data-language-local-name="林加拉语" class="interlanguage-link-target"><span>Lingála</span></a></li><li class="interlanguage-link interwiki-lt mw-list-item"><a href="https://lt.wikipedia.org/wiki/21_(skai%C4%8Dius)" title="21 (skaičius) – 立陶宛语" lang="lt" hreflang="lt" data-title="21 (skaičius)" data-language-autonym="Lietuvių" data-language-local-name="立陶宛语" class="interlanguage-link-target"><span>Lietuvių</span></a></li><li class="interlanguage-link interwiki-lv mw-list-item"><a href="https://lv.wikipedia.org/wiki/21_(skaitlis)" title="21 (skaitlis) – 拉脱维亚语" lang="lv" hreflang="lv" data-title="21 (skaitlis)" data-language-autonym="Latviešu" data-language-local-name="拉脱维亚语" class="interlanguage-link-target"><span>Latviešu</span></a></li><li class="interlanguage-link interwiki-mai mw-list-item"><a href="https://mai.wikipedia.org/wiki/%E0%A5%A8%E0%A5%A7" title="२१ – 迈蒂利语" lang="mai" hreflang="mai" data-title="२१" data-language-autonym="मैथिली" data-language-local-name="迈蒂利语" class="interlanguage-link-target"><span>मैथिली</span></a></li><li class="interlanguage-link interwiki-mk mw-list-item"><a href="https://mk.wikipedia.org/wiki/21_(%D0%B1%D1%80%D0%BE%D1%98)" title="21 (број) – 马其顿语" lang="mk" hreflang="mk" data-title="21 (број)" data-language-autonym="Македонски" data-language-local-name="马其顿语" class="interlanguage-link-target"><span>Македонски</span></a></li><li class="interlanguage-link interwiki-mni mw-list-item"><a href="https://mni.wikipedia.org/wiki/%EA%AF%B2%EA%AF%B1" title="꯲꯱ – 曼尼普尔语" lang="mni" hreflang="mni" data-title="꯲꯱" data-language-autonym="ꯃꯤꯇꯩ ꯂꯣꯟ" data-language-local-name="曼尼普尔语" class="interlanguage-link-target"><span>ꯃꯤꯇꯩ ꯂꯣꯟ</span></a></li><li class="interlanguage-link interwiki-mr mw-list-item"><a href="https://mr.wikipedia.org/wiki/%E0%A5%A8%E0%A5%A7_(%E0%A4%B8%E0%A4%82%E0%A4%96%E0%A5%8D%E0%A4%AF%E0%A4%BE)" title="२१ (संख्या) – 马拉地语" lang="mr" hreflang="mr" data-title="२१ (संख्या)" data-language-autonym="मराठी" data-language-local-name="马拉地语" class="interlanguage-link-target"><span>मराठी</span></a></li><li class="interlanguage-link interwiki-ms mw-list-item"><a href="https://ms.wikipedia.org/wiki/21_(nombor)" title="21 (nombor) – 马来语" lang="ms" hreflang="ms" data-title="21 (nombor)" data-language-autonym="Bahasa Melayu" data-language-local-name="马来语" class="interlanguage-link-target"><span>Bahasa Melayu</span></a></li><li class="interlanguage-link interwiki-myv mw-list-item"><a href="https://myv.wikipedia.org/wiki/21_(%D0%BB%D0%BE%D0%B2%D0%BE%D0%BC%D0%B0_%D0%B2%D0%B0%D0%BB)" title="21 (ловома вал) – 厄尔兹亚语" lang="myv" hreflang="myv" data-title="21 (ловома вал)" data-language-autonym="Эрзянь" data-language-local-name="厄尔兹亚语" class="interlanguage-link-target"><span>Эрзянь</span></a></li><li class="interlanguage-link interwiki-mzn mw-list-item"><a href="https://mzn.wikipedia.org/wiki/%DB%B2%DB%B1_(%D8%B9%D8%AF%D8%AF)" title="۲۱ (عدد) – 马赞德兰语" lang="mzn" hreflang="mzn" data-title="۲۱ (عدد)" data-language-autonym="مازِرونی" data-language-local-name="马赞德兰语" class="interlanguage-link-target"><span>مازِرونی</span></a></li><li class="interlanguage-link interwiki-nah mw-list-item"><a href="https://nah.wikipedia.org/wiki/C%C4%93mp%C5%8Dhualonc%C4%93" title="Cēmpōhualoncē – Nahuatl" lang="nah" hreflang="nah" data-title="Cēmpōhualoncē" data-language-autonym="Nāhuatl" data-language-local-name="Nahuatl" class="interlanguage-link-target"><span>Nāhuatl</span></a></li><li class="interlanguage-link interwiki-nap mw-list-item"><a href="https://nap.wikipedia.org/wiki/Vint%C3%B9ne" title="Vintùne – 那不勒斯语" lang="nap" hreflang="nap" data-title="Vintùne" data-language-autonym="Napulitano" data-language-local-name="那不勒斯语" class="interlanguage-link-target"><span>Napulitano</span></a></li><li class="interlanguage-link interwiki-nl mw-list-item"><a href="https://nl.wikipedia.org/wiki/21_(getal)" title="21 (getal) – 荷兰语" lang="nl" hreflang="nl" data-title="21 (getal)" data-language-autonym="Nederlands" data-language-local-name="荷兰语" class="interlanguage-link-target"><span>Nederlands</span></a></li><li class="interlanguage-link interwiki-nn mw-list-item"><a href="https://nn.wikipedia.org/wiki/Talet_21" title="Talet 21 – 挪威尼诺斯克语" lang="nn" hreflang="nn" data-title="Talet 21" data-language-autonym="Norsk nynorsk" data-language-local-name="挪威尼诺斯克语" class="interlanguage-link-target"><span>Norsk nynorsk</span></a></li><li class="interlanguage-link interwiki-no mw-list-item"><a href="https://no.wikipedia.org/wiki/21_(tall)" title="21 (tall) – 书面挪威语" lang="nb" hreflang="nb" data-title="21 (tall)" data-language-autonym="Norsk bokmål" data-language-local-name="书面挪威语" class="interlanguage-link-target"><span>Norsk bokmål</span></a></li><li class="interlanguage-link interwiki-nso mw-list-item"><a href="https://nso.wikipedia.org/wiki/21_(nomoro)" title="21 (nomoro) – 北索托语" lang="nso" hreflang="nso" data-title="21 (nomoro)" data-language-autonym="Sesotho sa Leboa" data-language-local-name="北索托语" class="interlanguage-link-target"><span>Sesotho sa Leboa</span></a></li><li class="interlanguage-link interwiki-pl mw-list-item"><a href="https://pl.wikipedia.org/wiki/21_(liczba)" title="21 (liczba) – 波兰语" lang="pl" hreflang="pl" data-title="21 (liczba)" data-language-autonym="Polski" data-language-local-name="波兰语" class="interlanguage-link-target"><span>Polski</span></a></li><li class="interlanguage-link interwiki-pnb mw-list-item"><a href="https://pnb.wikipedia.org/wiki/21" title="21 – Western Punjabi" lang="pnb" hreflang="pnb" data-title="21" data-language-autonym="پنجابی" data-language-local-name="Western Punjabi" class="interlanguage-link-target"><span>پنجابی</span></a></li><li class="interlanguage-link interwiki-ps mw-list-item"><a href="https://ps.wikipedia.org/wiki/%DB%B2%DB%B1_(%D8%B9%D8%AF%D8%AF)" title="۲۱ (عدد) – 普什图语" lang="ps" hreflang="ps" data-title="۲۱ (عدد)" data-language-autonym="پښتو" data-language-local-name="普什图语" class="interlanguage-link-target"><span>پښتو</span></a></li><li class="interlanguage-link interwiki-pt badge-Q70893996 mw-list-item" title=""><a href="https://pt.wikipedia.org/wiki/Vinte_e_um" title="Vinte e um – 葡萄牙语" lang="pt" hreflang="pt" data-title="Vinte e um" data-language-autonym="Português" data-language-local-name="葡萄牙语" class="interlanguage-link-target"><span>Português</span></a></li><li class="interlanguage-link interwiki-rn mw-list-item"><a href="https://rn.wikipedia.org/wiki/Mirongo_ibiri_na_rimwe" title="Mirongo ibiri na rimwe – 隆迪语" lang="rn" hreflang="rn" data-title="Mirongo ibiri na rimwe" data-language-autonym="Ikirundi" data-language-local-name="隆迪语" class="interlanguage-link-target"><span>Ikirundi</span></a></li><li class="interlanguage-link interwiki-ro mw-list-item"><a href="https://ro.wikipedia.org/wiki/21_(num%C4%83r)" title="21 (număr) – 罗马尼亚语" lang="ro" hreflang="ro" data-title="21 (număr)" data-language-autonym="Română" data-language-local-name="罗马尼亚语" class="interlanguage-link-target"><span>Română</span></a></li><li class="interlanguage-link interwiki-ru mw-list-item"><a href="https://ru.wikipedia.org/wiki/21_(%D1%87%D0%B8%D1%81%D0%BB%D0%BE)" title="21 (число) – 俄语" lang="ru" hreflang="ru" data-title="21 (число)" data-language-autonym="Русский" data-language-local-name="俄语" class="interlanguage-link-target"><span>Русский</span></a></li><li class="interlanguage-link interwiki-simple mw-list-item"><a href="https://simple.wikipedia.org/wiki/21_(number)" title="21 (number) – Simple English" lang="en-simple" hreflang="en-simple" data-title="21 (number)" data-language-autonym="Simple English" data-language-local-name="Simple English" class="interlanguage-link-target"><span>Simple English</span></a></li><li class="interlanguage-link interwiki-sl mw-list-item"><a href="https://sl.wikipedia.org/wiki/21_(%C5%A1tevilo)" title="21 (število) – 斯洛文尼亚语" lang="sl" hreflang="sl" data-title="21 (število)" data-language-autonym="Slovenščina" data-language-local-name="斯洛文尼亚语" class="interlanguage-link-target"><span>Slovenščina</span></a></li><li class="interlanguage-link interwiki-sm mw-list-item"><a href="https://sm.wikipedia.org/wiki/21_(numera)" title="21 (numera) – 萨摩亚语" lang="sm" hreflang="sm" data-title="21 (numera)" data-language-autonym="Gagana Samoa" data-language-local-name="萨摩亚语" class="interlanguage-link-target"><span>Gagana Samoa</span></a></li><li class="interlanguage-link interwiki-so mw-list-item"><a href="https://so.wikipedia.org/wiki/21_(tiro)" title="21 (tiro) – 索马里语" lang="so" hreflang="so" data-title="21 (tiro)" data-language-autonym="Soomaaliga" data-language-local-name="索马里语" class="interlanguage-link-target"><span>Soomaaliga</span></a></li><li class="interlanguage-link interwiki-sr mw-list-item"><a href="https://sr.wikipedia.org/wiki/21_(%D0%B1%D1%80%D0%BE%D1%98)" title="21 (број) – 塞尔维亚语" lang="sr" hreflang="sr" data-title="21 (број)" data-language-autonym="Српски / srpski" data-language-local-name="塞尔维亚语" class="interlanguage-link-target"><span>Српски / srpski</span></a></li><li class="interlanguage-link interwiki-srn mw-list-item"><a href="https://srn.wikipedia.org/wiki/Numro_21" title="Numro 21 – 苏里南汤加语" lang="srn" hreflang="srn" data-title="Numro 21" data-language-autonym="Sranantongo" data-language-local-name="苏里南汤加语" class="interlanguage-link-target"><span>Sranantongo</span></a></li><li class="interlanguage-link interwiki-sv mw-list-item"><a href="https://sv.wikipedia.org/wiki/21_(tal)" title="21 (tal) – 瑞典语" lang="sv" hreflang="sv" data-title="21 (tal)" data-language-autonym="Svenska" data-language-local-name="瑞典语" class="interlanguage-link-target"><span>Svenska</span></a></li><li class="interlanguage-link interwiki-sw mw-list-item"><a href="https://sw.wikipedia.org/wiki/Ishirini_na_moja" title="Ishirini na moja – 斯瓦希里语" lang="sw" hreflang="sw" data-title="Ishirini na moja" data-language-autonym="Kiswahili" data-language-local-name="斯瓦希里语" class="interlanguage-link-target"><span>Kiswahili</span></a></li><li class="interlanguage-link interwiki-th mw-list-item"><a href="https://th.wikipedia.org/wiki/21" title="21 – 泰语" lang="th" hreflang="th" data-title="21" data-language-autonym="ไทย" data-language-local-name="泰语" class="interlanguage-link-target"><span>ไทย</span></a></li><li class="interlanguage-link interwiki-tr mw-list-item"><a href="https://tr.wikipedia.org/wiki/21_(say%C4%B1)" title="21 (sayı) – 土耳其语" lang="tr" hreflang="tr" data-title="21 (sayı)" data-language-autonym="Türkçe" data-language-local-name="土耳其语" class="interlanguage-link-target"><span>Türkçe</span></a></li><li class="interlanguage-link interwiki-tt mw-list-item"><a href="https://tt.wikipedia.org/wiki/21_(%D1%81%D0%B0%D0%BD)" title="21 (сан) – 鞑靼语" lang="tt" hreflang="tt" data-title="21 (сан)" data-language-autonym="Татарча / tatarça" data-language-local-name="鞑靼语" class="interlanguage-link-target"><span>Татарча / tatarça</span></a></li><li class="interlanguage-link interwiki-tyv mw-list-item"><a href="https://tyv.wikipedia.org/wiki/21" title="21 – 图瓦语" lang="tyv" hreflang="tyv" data-title="21" data-language-autonym="Тыва дыл" data-language-local-name="图瓦语" class="interlanguage-link-target"><span>Тыва дыл</span></a></li><li class="interlanguage-link interwiki-uk mw-list-item"><a href="https://uk.wikipedia.org/wiki/21_(%D1%87%D0%B8%D1%81%D0%BB%D0%BE)" title="21 (число) – 乌克兰语" lang="uk" hreflang="uk" data-title="21 (число)" data-language-autonym="Українська" data-language-local-name="乌克兰语" class="interlanguage-link-target"><span>Українська</span></a></li><li class="interlanguage-link interwiki-ur mw-list-item"><a href="https://ur.wikipedia.org/wiki/21_(%D8%B9%D8%AF%D8%AF)" title="21 (عدد) – 乌尔都语" lang="ur" hreflang="ur" data-title="21 (عدد)" data-language-autonym="اردو" data-language-local-name="乌尔都语" class="interlanguage-link-target"><span>اردو</span></a></li><li class="interlanguage-link interwiki-uz mw-list-item"><a href="https://uz.wikipedia.org/wiki/21_(son)" title="21 (son) – 乌兹别克语" lang="uz" hreflang="uz" data-title="21 (son)" data-language-autonym="Oʻzbekcha / ўзбекча" data-language-local-name="乌兹别克语" class="interlanguage-link-target"><span>Oʻzbekcha / ўзбекча</span></a></li><li class="interlanguage-link interwiki-vep mw-list-item"><a href="https://vep.wikipedia.org/wiki/21_(lugu)" title="21 (lugu) – 维普森语" lang="vep" hreflang="vep" data-title="21 (lugu)" data-language-autonym="Vepsän kel’" data-language-local-name="维普森语" class="interlanguage-link-target"><span>Vepsän kel’</span></a></li><li class="interlanguage-link interwiki-vi mw-list-item"><a href="https://vi.wikipedia.org/wiki/21_(s%E1%BB%91)" title="21 (số) – 越南语" lang="vi" hreflang="vi" data-title="21 (số)" data-language-autonym="Tiếng Việt" data-language-local-name="越南语" class="interlanguage-link-target"><span>Tiếng Việt</span></a></li><li class="interlanguage-link interwiki-vls mw-list-item"><a href="https://vls.wikipedia.org/wiki/21_(getal)" title="21 (getal) – 西佛蘭德文" lang="vls" hreflang="vls" data-title="21 (getal)" data-language-autonym="West-Vlams" data-language-local-name="西佛蘭德文" class="interlanguage-link-target"><span>West-Vlams</span></a></li><li class="interlanguage-link interwiki-war mw-list-item"><a href="https://war.wikipedia.org/wiki/21_(ihap)" title="21 (ihap) – 瓦瑞语" lang="war" hreflang="war" data-title="21 (ihap)" data-language-autonym="Winaray" data-language-local-name="瓦瑞语" class="interlanguage-link-target"><span>Winaray</span></a></li><li class="interlanguage-link interwiki-wuu mw-list-item"><a href="https://wuu.wikipedia.org/wiki/21" title="21 – 吴语" lang="wuu" hreflang="wuu" data-title="21" data-language-autonym="吴语" data-language-local-name="吴语" class="interlanguage-link-target"><span>吴语</span></a></li><li class="interlanguage-link interwiki-xh mw-list-item"><a href="https://xh.wikipedia.org/wiki/Amashumi_amabini_ananye" title="Amashumi amabini ananye – 科萨语" lang="xh" hreflang="xh" data-title="Amashumi amabini ananye" data-language-autonym="IsiXhosa" data-language-local-name="科萨语" class="interlanguage-link-target"><span>IsiXhosa</span></a></li><li class="interlanguage-link interwiki-yi mw-list-item"><a href="https://yi.wikipedia.org/wiki/21_(%D7%A0%D7%95%D7%9E%D7%A2%D7%A8)" title="21 (נומער) – 意第绪语" lang="yi" hreflang="yi" data-title="21 (נומער)" data-language-autonym="ייִדיש" data-language-local-name="意第绪语" class="interlanguage-link-target"><span>ייִדיש</span></a></li><li class="interlanguage-link interwiki-za mw-list-item"><a href="https://za.wikipedia.org/wiki/21" title="21 – 壮语" lang="za" hreflang="za" data-title="21" data-language-autonym="Vahcuengh" data-language-local-name="壮语" class="interlanguage-link-target"><span>Vahcuengh</span></a></li><li class="interlanguage-link interwiki-zh-min-nan mw-list-item"><a href="https://zh-min-nan.wikipedia.org/wiki/21" title="21 – 闽南语" lang="nan" hreflang="nan" data-title="21" data-language-autonym="閩南語 / Bân-lâm-gú" data-language-local-name="闽南语" class="interlanguage-link-target"><span>閩南語 / Bân-lâm-gú</span></a></li><li class="interlanguage-link interwiki-zh-yue mw-list-item"><a href="https://zh-yue.wikipedia.org/wiki/21" title="21 – 粤语" lang="yue" hreflang="yue" data-title="21" data-language-autonym="粵語" data-language-local-name="粤语" class="interlanguage-link-target"><span>粵語</span></a></li> </ul> <div class="after-portlet 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href="/w/index.php?title=21&action=history"><span>查看历史</span></a></li> </ul> </div> </div> <div id="p-tb" class="vector-menu mw-portlet mw-portlet-tb" > <div class="vector-menu-heading"> 常规 </div> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li id="t-whatlinkshere" class="mw-list-item"><a href="/wiki/Special:%E9%93%BE%E5%85%A5%E9%A1%B5%E9%9D%A2/21" title="列出所有与本页相链的页面[j]" accesskey="j"><span>链入页面</span></a></li><li id="t-recentchangeslinked" class="mw-list-item"><a href="/wiki/Special:%E9%93%BE%E5%87%BA%E6%9B%B4%E6%94%B9/21" rel="nofollow" title="页面链出所有页面的更改[k]" accesskey="k"><span>相关更改</span></a></li><li id="t-upload" class="mw-list-item"><a href="//zh.wikipedia.org/wiki/Project:上传" title="上传图像或多媒体文件[u]" accesskey="u"><span>上传文件</span></a></li><li id="t-permalink" class="mw-list-item"><a href="/w/index.php?title=21&oldid=86337288" title="此页面该修订版本的固定链接"><span>固定链接</span></a></li><li id="t-info" class="mw-list-item"><a href="/w/index.php?title=21&action=info" title="关于此页面的更多信息"><span>页面信息</span></a></li><li id="t-cite" class="mw-list-item"><a href="/w/index.php?title=Special:%E5%BC%95%E7%94%A8%E6%AD%A4%E9%A1%B5%E9%9D%A2&page=21&id=86337288&wpFormIdentifier=titleform" title="有关如何引用此页面的信息"><span>引用此页</span></a></li><li id="t-urlshortener" class="mw-list-item"><a href="/w/index.php?title=Special:URL%E7%BC%A9%E7%9F%AD%E7%A8%8B%E5%BA%8F&url=https%3A%2F%2Fzh.wikipedia.org%2Fwiki%2F21"><span>获取短链接</span></a></li><li id="t-urlshortener-qrcode" class="mw-list-item"><a href="/w/index.php?title=Special:QrCode&url=https%3A%2F%2Fzh.wikipedia.org%2Fwiki%2F21"><span>下载二维码</span></a></li> </ul> </div> </div> <div id="p-electronpdfservice-sidebar-portlet-heading" class="vector-menu mw-portlet mw-portlet-electronpdfservice-sidebar-portlet-heading" > <div class="vector-menu-heading"> 打印/导出 </div> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li id="electron-print_pdf" class="mw-list-item"><a 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class="vector-column-end"> <div class="vector-sticky-pinned-container"> <nav class="vector-page-tools-landmark" aria-label="页面工具"> <div id="vector-page-tools-pinned-container" class="vector-pinned-container"> </div> </nav> <nav class="vector-appearance-landmark" aria-label="外观"> <div id="vector-appearance-pinned-container" class="vector-pinned-container"> <div id="vector-appearance" class="vector-appearance vector-pinnable-element"> <div class="vector-pinnable-header vector-appearance-pinnable-header vector-pinnable-header-pinned" data-feature-name="appearance-pinned" data-pinnable-element-id="vector-appearance" data-pinned-container-id="vector-appearance-pinned-container" data-unpinned-container-id="vector-appearance-unpinned-container" > <div class="vector-pinnable-header-label">外观</div> <button class="vector-pinnable-header-toggle-button vector-pinnable-header-pin-button" data-event-name="pinnable-header.vector-appearance.pin">移至侧栏</button> <button class="vector-pinnable-header-toggle-button vector-pinnable-header-unpin-button" data-event-name="pinnable-header.vector-appearance.unpin">隐藏</button> </div> </div> </div> </nav> </div> </div> <div id="bodyContent" class="vector-body" aria-labelledby="firstHeading" data-mw-ve-target-container> <div class="vector-body-before-content"> <div class="mw-indicators"> <div id="mw-indicator-noteTA-307af9e9" class="mw-indicator"><div class="mw-parser-output"><span class="skin-invert" typeof="mw:File"><span title="本页使用了标题或全文手工转换"><img alt="本页使用了标题或全文手工转换" src="//upload.wikimedia.org/wikipedia/commons/thumb/c/cd/Zh_conversion_icon_m.svg/35px-Zh_conversion_icon_m.svg.png" decoding="async" width="35" height="22" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/c/cd/Zh_conversion_icon_m.svg/53px-Zh_conversion_icon_m.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/c/cd/Zh_conversion_icon_m.svg/70px-Zh_conversion_icon_m.svg.png 2x" data-file-width="32" data-file-height="20" /></span></span></div></div> </div> <div id="siteSub" class="noprint">维基百科，自由的百科全书</div> </div> <div id="contentSub"><div id="mw-content-subtitle"></div></div> <div id="mw-content-text" class="mw-body-content"><div class="mw-content-ltr mw-parser-output" lang="zh" dir="ltr"><div id="noteTA-307af9e9" class="noteTA"><div class="noteTA-group"><div data-noteta-group-source="module" data-noteta-group="Math"></div></div></div> <style data-mw-deduplicate="TemplateStyles:r85100532">.mw-parser-output .hatnote{font-size:small}.mw-parser-output div.hatnote{padding-left:2em;margin-bottom:0.8em;margin-top:0.8em}.mw-parser-output .hatnote-notice-img::after{content:"\202f \202f \202f \202f "}.mw-parser-output .hatnote-notice-img-small::after{content:"\202f \202f "}.mw-parser-output .hatnote+link+.hatnote{margin-top:-0.5em}body.skin-minerva .mw-parser-output .hatnote-notice-img,body.skin-minerva .mw-parser-output .hatnote-notice-img-small{display:none}@media print{body.ns-0 .mw-parser-output .hatnote{display:none!important}}</style><div role="note" class="hatnote navigation-not-searchable"><span class="noviewer hatnote-notice-img" typeof="mw:File"><a href="/wiki/Wikipedia:%E6%B6%88%E6%AD%A7%E4%B9%89" title="Wikipedia:消歧义"><img alt="" src="//upload.wikimedia.org/wikipedia/commons/thumb/5/5f/Disambig_gray.svg/25px-Disambig_gray.svg.png" decoding="async" width="25" height="19" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/5/5f/Disambig_gray.svg/38px-Disambig_gray.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/5/5f/Disambig_gray.svg/50px-Disambig_gray.svg.png 2x" data-file-width="220" data-file-height="168" /></a></span>此条目的主題是整數。关于<a href="/wiki/%E8%8B%B1%E5%9C%8B" class="mw-redirect" title="英國">英國</a>女歌手<a href="/wiki/Adele" class="mw-redirect" title="Adele">Adele</a>在2011年發行的專輯，請見「<b><a href="/wiki/21_(%E5%B0%88%E8%BC%AF)" class="mw-redirect" title="21 (專輯)">21 (專輯)</a></b>」。</div> <style data-mw-deduplicate="TemplateStyles:r67735281">body.skin-minerva .mw-parser-output .infobox table{display:table}body.skin-minerva .mw-parser-output .infobox caption{display:table-caption}</style><style data-mw-deduplicate="TemplateStyles:r83732082">.mw-parser-output .infobox-subbox{padding:0;border:none;margin:-3px;width:auto;min-width:100%;font-size:100%;clear:none;float:none;background-color:transparent}.mw-parser-output .infobox-3cols-child{margin:auto}.mw-parser-output .infobox .navbar{font-size:100%}body.skin-minerva .mw-parser-output .infobox-header,body.skin-minerva .mw-parser-output .infobox-subheader,body.skin-minerva .mw-parser-output .infobox-above,body.skin-minerva .mw-parser-output .infobox-title,body.skin-minerva .mw-parser-output .infobox-image,body.skin-minerva .mw-parser-output .infobox-full-data,body.skin-minerva .mw-parser-output .infobox-below{text-align:center}@media screen{html.skin-theme-clientpref-night .mw-parser-output .infobox-full-data:not(.notheme)>div:not(.notheme)[style]{background:#1f1f23!important;color:#f8f9fa}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .infobox-full-data:not(.notheme) div:not(.notheme){background:#1f1f23!important;color:#f8f9fa}}html.skin-theme-clientpref-night .mw-parser-output .infobox td div:not(.notheme)[style]{background:transparent!important;color:var(--color-base,#202122)}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .infobox td div:not(.notheme)[style]{background:transparent!important;color:var(--color-base,#202122)}}html.skin-theme-clientpref-night .mw-parser-output .infobox td div.NavHead:not(.notheme)[style]{background:transparent!important}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .infobox td div.NavHead:not(.notheme)[style]{background:transparent!important}}@media(min-width:640px){body.skin--responsive .mw-parser-output .infobox-table{display:table!important}body.skin--responsive .mw-parser-output .infobox-table>caption{display:table-caption!important}body.skin--responsive .mw-parser-output .infobox-table>tbody{display:table-row-group}body.skin--responsive .mw-parser-output .infobox-table tr{display:table-row!important}body.skin--responsive .mw-parser-output .infobox-table th,body.skin--responsive .mw-parser-output .infobox-table td{padding-left:inherit;padding-right:inherit}}</style><table class="infobox" style="line-height: 1.5em"><caption class="infobox-title"><style data-mw-deduplicate="TemplateStyles:r85009877">.mw-parser-output .hvanchor>:target~.hvanchor-text{display:initial}.mw-parser-output .hvanchor>:target~.svanchor-text,.mw-parser-output .hvanchor>:target~.svanchor-text .anchor-high-light,.mw-parser-output .hvanchor>:target~.hvanchor-text .anchor-high-light{background-color:#b1d2ff}.mw-parser-output .hvanchor-text{display:none}.mw-parser-output .hvanchor>:target~.ihvanchor-text{display:none}.mw-parser-output .ihvanchor-text{display:initial}</style><span class="hvanchor"><span class="anchor" id="21資料"></span><span class="anchor" id="21資料"></span><span class="svanchor-text">21</span><span class="hvanchor-text"></span></span></caption><tbody><tr><th colspan="2" class="infobox-above" style="font-size: 150%"><table style="width:100%; margin:0"><tbody><tr> <td style="width:15%; text-align:left; white-space: nowrap; font-size:smaller"><a href="/wiki/20" title="20">← 20 </a></td> <td style="width:70%; padding-left:1em; padding-right:1em; text-align: center;">21</td> <td style="width:15%; text-align:right; white-space: nowrap; font-size:smaller"><a href="/wiki/22" title="22"> 22 →</a></td> </tr></tbody></table></th></tr><tr><td colspan="2" class="infobox-subheader" style="font-size:100%;"><table style="width:100%; margin:0;"><tbody><tr style="align:center;white-space:nowrap;"> <td><a href="/wiki/%E6%95%B0%E8%A1%A8" class="mw-redirect" title="数表">数表</a> — <a href="/wiki/%E6%95%B4%E6%95%B0" title="整数">整数</a> <div style="text−align: center;"> <p><small> <a href="/wiki/19" title="19"><<</a>  <a href="/wiki/20" title="20">20</a> <a class="mw-selflink selflink">21</a> <a href="/wiki/22" title="22">22</a> <a href="/wiki/23" title="23">23</a> <a href="/wiki/24" title="24">24</a> <a href="/wiki/25" title="25">25</a> <a href="/wiki/26" title="26">26</a> <a href="/wiki/27" title="27">27</a> <a href="/wiki/28" title="28">28</a> <a href="/wiki/29" title="29">29</a> <a href="/wiki/30" title="30">>></a> </small> </p> </div></td> </tr></tbody></table></td></tr><tr><th colspan="2" class="infobox-header" style="">命名</th></tr><tr><th scope="row" class="infobox-label" style=""><a href="/wiki/%E4%B8%AD%E6%96%87%E6%95%B8%E5%AD%97" class="mw-redirect" title="中文數字">小寫</a></th><td class="infobox-data" style="">二十一</td></tr><tr><th scope="row" class="infobox-label" style=""><a href="/wiki/%E4%B8%AD%E6%96%87%E6%95%B8%E5%AD%97" class="mw-redirect" title="中文數字">大寫</a></th><td class="infobox-data" style="">貳拾壹</td></tr><tr><th scope="row" class="infobox-label" style=""><a href="/wiki/%E5%BA%8F%E6%95%B8%E8%A9%9E" title="序數詞">序數詞</a></th><td class="infobox-data" style="">第二十一<br />twenty-first</td></tr><tr><th colspan="2" class="infobox-header" style="">識別</th></tr><tr><th scope="row" class="infobox-label" style="">種類</th><td class="infobox-data" style="">整數</td></tr><tr><th colspan="2" class="infobox-header" style="">性質</th></tr><tr><th scope="row" class="infobox-label" style=""><span class="nowrap"><a href="/wiki/%E8%B3%AA%E5%9B%A0%E6%95%B8%E5%88%86%E8%A7%A3" class="mw-redirect" title="質因數分解">質因數分解</a></span></th><td class="infobox-data" style=""><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle }"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"></mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle }</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/df4dcd61276328f7c7ec5bdc399b6e11114a2c68" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.171ex; width:0; height:0.343ex;" alt="{\displaystyle }" /></span><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle 3\times 7}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mn>3</mn> <mo>×</mo> <mn>7</mn> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle 3\times 7}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/215c3ee867a09f67ab795b7800d29cdd5966fd0e" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:5.165ex; height:2.176ex;" alt="{\displaystyle 3\times 7}" /></span></td></tr><tr><th colspan="2" class="infobox-header" style="">表示方式</th></tr><tr><th scope="row" class="infobox-label" style=""><a href="/wiki/%E5%80%BC" title="值">值</a></th><td class="infobox-data" style="">21</td></tr><tr><th scope="row" class="infobox-label" style=""><a href="/wiki/%E7%AE%97%E7%AD%B9" title="算筹">算筹</a></th><td class="infobox-data" style=""><span class="mw-default-size" typeof="mw:File"><a href="/wiki/File:Counting_rod_h2.png" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/5/51/Counting_rod_h2.png" decoding="async" width="29" height="13" class="mw-file-element" data-file-width="29" data-file-height="13" /></a></span><span class="mw-default-size" typeof="mw:File"><a href="/wiki/File:Counting_rod_v1.png" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/1/19/Counting_rod_v1.png" decoding="async" width="7" height="29" class="mw-file-element" data-file-width="7" data-file-height="29" /></a></span></td></tr><tr><th scope="row" class="infobox-label" style=""><a href="/wiki/%E5%B8%8C%E8%85%8A%E6%95%B0%E5%AD%97" title="希腊数字">希腊数字</a></th><td class="infobox-data" style=""><span lang="el">ΚΑ´ </span></td></tr><tr><th scope="row" class="infobox-label" style=""><a href="/wiki/%E7%BE%85%E9%A6%AC%E6%95%B8%E5%AD%97" class="mw-redirect" title="羅馬數字">羅馬數字</a></th><td class="infobox-data" style=""><style data-mw-deduplicate="TemplateStyles:r75010853">.mw-parser-output .roman-numeral{font-family:"Nimbus Roman No9 L","Times New Roman",Times,serif;font-size:118%;line-height:1}.mw-parser-output .roman-numeral-a{border:1px solid}.mw-parser-output .roman-numeral-t{border-top:1px solid}.mw-parser-output .roman-numeral-v{border:solid;border-width:0 1px;padding:0 2px}.mw-parser-output .roman-numeral-h{border:solid;border-width:1px 0}.mw-parser-output .roman-numeral-tv{border:1px solid;border-bottom:none;padding:0 2px}</style><span class="roman-numeral" title="Roman numeral">XXI </span></td></tr><tr><td colspan="2" class="infobox-full-data" style=""></td></tr><tr><th><a href="/wiki/%E5%B7%B4%E6%AF%94%E4%BC%A6%E6%95%B0%E5%AD%97" title="巴比伦数字">巴比伦数字</a></th><td>𒎙𒐕<span style="float:right"><span class="mw-valign-text-top" typeof="mw:File/Frameless"><a href="https://www.wikidata.org/wiki/Q712477?uselang=zh#Q712477$C89FCEED-D549-4732-8E97-551B85E62166" title="在维基数据编辑"><img alt="在维基数据编辑" src="//upload.wikimedia.org/wikipedia/commons/thumb/8/8a/OOjs_UI_icon_edit-ltr-progressive.svg/10px-OOjs_UI_icon_edit-ltr-progressive.svg.png" decoding="async" width="10" height="10" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/8/8a/OOjs_UI_icon_edit-ltr-progressive.svg/15px-OOjs_UI_icon_edit-ltr-progressive.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/8/8a/OOjs_UI_icon_edit-ltr-progressive.svg/20px-OOjs_UI_icon_edit-ltr-progressive.svg.png 2x" data-file-width="20" data-file-height="20" /></a></span></span></td></tr><tr style="display:none"><td colspan="2"> </td></tr><tr><td colspan="2" class="infobox-full-data" style=""></td></tr><tr><th><a href="/wiki/%E4%BA%8C%E8%BF%9B%E5%88%B6" title="二进制">二进制</a></th><td>10101<sub>(2)</sub></td></tr><tr><th><a href="/wiki/%E4%B8%89%E8%BF%9B%E5%88%B6" title="三进制">三进制</a></th><td>210<sub>(3)</sub></td></tr><tr><th><a href="/wiki/%E5%9B%9B%E8%BF%9B%E5%88%B6" class="mw-redirect" title="四进制">四进制</a></th><td>111<sub>(4)</sub></td></tr><tr><th><a href="/wiki/%E4%BA%94%E9%80%B2%E5%88%B6" title="五進制">五进制</a></th><td>41<sub>(5)</sub></td></tr><tr><th><a href="/wiki/%E5%85%AB%E8%BF%9B%E5%88%B6" title="八进制">八进制</a></th><td>25<sub>(8)</sub></td></tr><tr><th><a href="/wiki/%E5%8D%81%E4%BA%8C%E8%BF%9B%E5%88%B6" title="十二进制">十二进制</a></th><td>19<sub>(12)</sub></td></tr><tr><th><a href="/wiki/%E5%8D%81%E5%85%AD%E8%BF%9B%E5%88%B6" title="十六进制">十六进制</a></th><td>15<sub>(16)</sub></td></tr><tr style="display:none"><td colspan="2"> </td></tr><tr><td colspan="2" class="infobox-navbar"><style data-mw-deduplicate="TemplateStyles:r84265675">.mw-parser-output .hlist dl,.mw-parser-output .hlist ol,.mw-parser-output .hlist ul{margin:0;padding:0}.mw-parser-output .hlist dd,.mw-parser-output .hlist dt,.mw-parser-output .hlist li{margin:0;display:inline}.mw-parser-output .hlist.inline,.mw-parser-output .hlist.inline dl,.mw-parser-output .hlist.inline ol,.mw-parser-output .hlist.inline ul,.mw-parser-output .hlist dl dl,.mw-parser-output .hlist dl ol,.mw-parser-output .hlist dl ul,.mw-parser-output .hlist ol dl,.mw-parser-output .hlist ol ol,.mw-parser-output .hlist ol ul,.mw-parser-output .hlist ul dl,.mw-parser-output .hlist ul ol,.mw-parser-output .hlist ul ul{display:inline}.mw-parser-output .hlist .mw-empty-li{display:none}.mw-parser-output .hlist dt::after{content:" :"}.mw-parser-output .hlist dd::after,.mw-parser-output .hlist li::after{content:" · ";font-weight:bold}.mw-parser-output .hlist-pipe dd::after,.mw-parser-output .hlist-pipe li::after{content:" | ";font-weight:normal}.mw-parser-output .hlist-hyphen dd::after,.mw-parser-output .hlist-hyphen li::after{content:" - ";font-weight:normal}.mw-parser-output .hlist-comma dd::after,.mw-parser-output .hlist-comma li::after{content:"、";font-weight:normal}.mw-parser-output .hlist dd:last-child::after,.mw-parser-output .hlist dt:last-child::after,.mw-parser-output .hlist li:last-child::after{content:none}.mw-parser-output .hlist ol{counter-reset:listitem}.mw-parser-output .hlist ol>li{counter-increment:listitem}.mw-parser-output .hlist ol>li::before{content:" "counter(listitem)"\a0 "}.mw-parser-output .hlist dd ol>li:first-child::before,.mw-parser-output .hlist dt ol>li:first-child::before,.mw-parser-output .hlist li ol>li:first-child::before{content:"（"counter(listitem)"\a0 "}.mw-parser-output ul.cslist,.mw-parser-output ul.sslist{margin:0;padding:0;display:inline-block;list-style:none}.mw-parser-output .cslist li,.mw-parser-output .sslist li{margin:0;display:inline-block}.mw-parser-output .cslist li::after{content:"，"}.mw-parser-output .sslist li::after{content:"；"}.mw-parser-output .cslist li:last-child::after,.mw-parser-output .sslist li:last-child::after{content:none}</style><style data-mw-deduplicate="TemplateStyles:r84244141">.mw-parser-output .navbar{display:inline;font-weight:normal}.mw-parser-output .navbar-collapse{float:left;text-align:left}.mw-parser-output .navbar-boxtext{word-spacing:0}.mw-parser-output .navbar ul{display:inline-block;white-space:nowrap;line-height:inherit}.mw-parser-output .navbar-brackets::before{margin-right:-0.125em;content:"[ "}.mw-parser-output .navbar-brackets::after{margin-left:-0.125em;content:" ]"}.mw-parser-output .navbar li{word-spacing:-0.125em}.mw-parser-output .navbar a>span,.mw-parser-output .navbar a>abbr{text-decoration:inherit}.mw-parser-output .navbar-mini abbr{font-variant:small-caps;border-bottom:none;text-decoration:none;cursor:inherit}.mw-parser-output .navbar-ct-full{font-size:110%;margin:0 8em}.mw-parser-output .navbar-ct-mini{font-size:110%;margin:0 5em}html.skin-theme-clientpref-night .mw-parser-output .navbar li a abbr{color:var(--color-base)!important}@media(prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .navbar li a abbr{color:var(--color-base)!important}}@media print{.mw-parser-output .navbar{display:none!important}}</style><div class="navbar plainlinks hlist navbar-mini"><ul><li class="nv-view"><a href="/wiki/Template:%E6%95%B4%E6%95%B0" title="Template:整数"><abbr title="查看该模板">查</abbr></a></li><li class="nv-talk"><a href="/wiki/Template_talk:%E6%95%B4%E6%95%B0" title="Template talk:整数"><abbr title="讨论该模板">论</abbr></a></li><li class="nv-edit"><a href="/wiki/Special:%E7%BC%96%E8%BE%91%E9%A1%B5%E9%9D%A2/Template:%E6%95%B4%E6%95%B0" title="Special:编辑页面/Template:整数"><abbr title="编辑该模板">编</abbr></a></li></ul></div></td></tr></tbody></table> <p><b>21</b>（二十一）是<a href="/wiki/20" title="20">20</a>与<a href="/wiki/22" title="22">22</a>之间的<a href="/wiki/%E8%87%AA%E7%84%B6%E6%95%B0" title="自然数">自然数</a>。 </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="数学性质"><span id=".E6.95.B0.E5.AD.A6.E6.80.A7.E8.B4.A8"></span>数学性质</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=21&action=edit&section=1" title="编辑章节：数学性质"><span>编辑</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li>第12個<a href="/wiki/%E5%90%88%E6%95%B0" title="合数">合數</a>，<a href="/wiki/%E5%9B%A0%E6%95%B8" title="因數">正因數</a>有1、3、7和21。前一個為<a href="/wiki/20" title="20">20</a>、下一個為<a href="/wiki/22" title="22">22</a>。 <dl><dd><a href="/wiki/%E6%95%B4%E6%95%B0%E5%88%86%E8%A7%A3" title="整数分解">質因數分解</a>為<span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle 3\times 7}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mn>3</mn> <mo>×</mo> <mn>7</mn> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle 3\times 7}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/215c3ee867a09f67ab795b7800d29cdd5966fd0e" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:5.165ex; height:2.176ex;" alt="{\displaystyle 3\times 7}" /></span>。</dd></dl></li> <li>第17個<a href="/wiki/%E4%BA%8F%E6%95%B0" title="亏数">虧數</a>，<a href="/wiki/%E7%9C%9F%E5%9B%A0%E6%95%B8%E5%92%8C" title="真因數和">真因數和</a>為11，虧度為10。前一個為<a href="/wiki/19" title="19">19</a>、下一個為<a href="/wiki/22" title="22">22</a>。</li> <li>第14個<a href="/wiki/%E4%B8%8D%E5%B0%8B%E5%B8%B8%E6%95%B8" title="不尋常數">不尋常數</a>，大於平方根的質因數為7。前一個為<a href="/wiki/20" title="20">20</a>、下一個為<a href="/wiki/22" title="22">22</a>。</li> <li>第7個<a href="/wiki/%E5%8D%8A%E7%B4%A0%E6%95%B0" title="半素数">半質數</a>。前一個為<a href="/wiki/15" title="15">15</a>、下一個為<a href="/wiki/22" title="22">22</a>。</li> <li>第14個<a href="/wiki/%E6%97%A0%E5%B9%B3%E6%96%B9%E6%95%B0%E5%9B%A0%E6%95%B0%E7%9A%84%E6%95%B0" class="mw-redirect" title="无平方数因数的数">無平方數因數的數</a>。前一個為<a href="/wiki/19" title="19">19</a>、下一個為<a href="/wiki/22" title="22">22</a>。</li> <li>第8個<a href="/wiki/%E6%96%90%E6%B3%A2%E9%82%A3%E5%A5%91%E6%95%B0%E5%88%97" class="mw-redirect" title="斐波那契数列">斐波那契數</a>。前一個為<a href="/wiki/13" title="13">13</a>、下一個為<a href="/wiki/34" title="34">34</a>。</li> <li>第14個<a href="/wiki/%E5%8D%81%E8%BF%9B%E5%88%B6" title="十进制">十进制</a>的<a href="/wiki/%E5%93%88%E6%B2%99%E5%BE%B7%E6%95%B8" title="哈沙德數">哈沙德數</a>。前一個為<a href="/wiki/20" title="20">20</a>、下一個為<a href="/wiki/24" title="24">24</a>。</li> <li>第14個<a href="/wiki/%E5%8D%81%E8%BF%9B%E5%88%B6" title="十进制">十进制</a>的<a href="/wiki/%E7%AD%89%E6%95%B8%E4%BD%8D%E6%95%B8" title="等數位數">等數位數</a>。前一個為<a href="/wiki/19" title="19">19</a>、下一個為<a href="/wiki/23" title="23">23</a>。</li> <li><a href="/wiki/%E5%A4%96%E8%A7%80%E6%95%B8%E5%88%97" title="外觀數列">外觀數列</a>的第3項</li></ul> <figure class="mw-default-size mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:SQSQ21.gif" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/b/bd/SQSQ21.gif/220px-SQSQ21.gif" decoding="async" width="220" height="220" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/b/bd/SQSQ21.gif 1.5x" data-file-width="224" data-file-height="224" /></a><figcaption>最小的完美正方形</figcaption></figure> <ul><li>第6個<a href="/wiki/%E4%B8%89%E8%A7%92%E5%BD%A2%E6%95%B8" title="三角形數">三角形數</a>、第3個<a href="/wiki/%E5%85%AB%E9%82%8A%E5%BD%A2%E6%95%B8" title="八邊形數">八邊形數</a></li> <li><a href="/wiki/%E9%BB%98%E6%85%88%E9%87%91%E6%95%B8" title="默慈金數">默慈金數</a></li> <li><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {{508853989}^{2}}=258932382121212121}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <msup> <mrow class="MJX-TeXAtom-ORD"> <mn>508853989</mn> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msup> </mrow> <mo>=</mo> <mn>258932382121212121</mn> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle {{508853989}^{2}}=258932382121212121}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/a627759d2c9e7f2e960aad410de62eb00c44f2d9" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:35.539ex; height:2.676ex;" alt="{\displaystyle {{508853989}^{2}}=258932382121212121}" /></span></li> <li>最小的<a href="/wiki/%E5%AE%8C%E7%BE%8E%E6%AD%A3%E6%96%B9%E5%BD%A2" title="完美正方形">完美正方形</a>由21個小<a href="/wiki/%E6%AD%A3%E6%96%B9%E5%BD%A2" title="正方形">正方形</a>組成</li></ul> <div class="mw-heading mw-heading3"><h3 id="基本运算"><span id=".E5.9F.BA.E6.9C.AC.E8.BF.90.E7.AE.97"></span>基本运算</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=21&action=edit&section=2" title="编辑章节：基本运算"><span>编辑</span></a><span class="mw-editsection-bracket">]</span></span></div> <table class="wikitable"> <tbody><tr> <th><a href="/wiki/%E4%B9%98%E6%B3%95" title="乘法">乘法</a> </th> <th>1 </th> <th>2 </th> <th>3 </th> <th>4 </th> <th>5 </th> <th>6 </th> <th>7 </th> <th>8 </th> <th>9 </th> <th>10 </th> <th width="5px"> </th> <th>11 </th> <th>12 </th> <th>13 </th> <th>14 </th> <th>15 </th> <th>16 </th> <th>17 </th> <th>18 </th> <th>19 </th> <th>20 </th> <th width="5px"> </th> <th>21 </th> <th>22 </th> <th>23 </th> <th>24 </th> <th>25 </th></tr> <tr> <th><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {{21}\times {x}}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mrow class="MJX-TeXAtom-ORD"> <mn>21</mn> </mrow> <mo>×</mo> <mrow class="MJX-TeXAtom-ORD"> <mi>x</mi> </mrow> </mrow> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle {{21}\times {x}}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/5d4a8028142f0a0f745c09390ff29b28e65c7026" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:6.495ex; height:2.176ex;" alt="{\displaystyle {{21}\times {x}}}" /></span> </th> <td><a class="mw-selflink selflink">21</a> </td> <td><a href="/wiki/42" title="42">42</a> </td> <td><a href="/wiki/63" title="63">63</a> </td> <td><a href="/wiki/84" title="84">84</a> </td> <td><a href="/wiki/105" title="105">105</a> </td> <td><a href="/wiki/126" title="126">126</a> </td> <td><a href="/wiki/147" title="147">147</a> </td> <td><a href="/wiki/168" title="168">168</a> </td> <td><a href="/wiki/189" title="189">189</a> </td> <td><a href="/wiki/210" title="210">210</a> </td> <th width="5px"> </th> <td><a href="/wiki/231" title="231">231</a> </td> <td><a href="/wiki/252" title="252">252</a> </td> <td><a href="/wiki/273" title="273">273</a> </td> <td><a href="/wiki/294" title="294">294</a> </td> <td><a href="/wiki/315" title="315">315</a> </td> <td><a href="/wiki/336" class="mw-redirect" title="336">336</a> </td> <td><a href="/wiki/357" class="mw-redirect" title="357">357</a> </td> <td><a href="/wiki/378" class="mw-redirect" title="378">378</a> </td> <td><a href="/wiki/399" class="mw-redirect" title="399">399</a> </td> <td><a href="/wiki/420" class="mw-redirect" title="420">420</a> </td> <th width="5px"> </th> <td><a href="/wiki/441" class="mw-redirect" title="441">441</a> </td> <td><a href="/wiki/462" class="mw-redirect" title="462">462</a> </td> <td><a href="/wiki/483" class="mw-redirect" title="483">483</a> </td> <td><a href="/wiki/504" class="mw-redirect" title="504">504</a> </td> <td><a href="/w/index.php?title=525&action=edit&redlink=1" class="new" title="525（页面不存在）">525</a> </td></tr></tbody></table> <div class="mw-heading mw-heading2"><h2 id="在科学中"><span id=".E5.9C.A8.E7.A7.91.E5.AD.A6.E4.B8.AD"></span>在科学中</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=21&action=edit&section=3" title="编辑章节：在科学中"><span>编辑</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/%E9%88%A7" class="mw-redirect" title="鈧">鈧</a>的<a href="/wiki/%E5%8E%9F%E5%AD%90%E5%BA%8F%E6%95%B8" class="mw-redirect" title="原子序數">原子序數</a><sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup></li></ul> <div class="mw-heading mw-heading2"><h2 id="在人类文化中"><span id=".E5.9C.A8.E4.BA.BA.E7.B1.BB.E6.96.87.E5.8C.96.E4.B8.AD"></span>在人类文化中</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=21&action=edit&section=4" title="编辑章节：在人类文化中"><span>编辑</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/%E5%BB%BF%E4%B8%80%E9%BB%9E" class="mw-redirect" title="廿一點">廿一點</a>是一種牌類遊戲。</li> <li>俗語「三七二十一」指不顧一切。<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup></li> <li>在<a href="/wiki/Key_(%E5%85%AC%E5%8F%B8)" class="mw-redirect" title="Key (公司)">Key</a>公司的<a href="/wiki/%E7%BE%8E%E5%B0%91%E5%A5%B3%E9%81%8A%E6%88%B2" title="美少女遊戲">美少女遊戲</a><a href="/wiki/Little_Busters!" title="Little Busters!">Little Busters!</a>中，其角色棗恭介曾經提到「<b>(21)</b>」若快寫的話，就會變成「<b><span lang="ja">ロリ</span></b>」（<span lang="ja">ロリ</span>即日語的<a href="/wiki/%E8%98%BF%E8%8E%89" title="蘿莉">蘿莉</a>）之事。</li> <li>在美國某些地方，21歲以上才能賭博。</li> <li><a href="/wiki/%E5%95%A4%E9%85%92" title="啤酒">啤酒</a>瓶蓋的鋸齒數量。</li> <li>在<a href="/wiki/%E5%8F%B0%E7%81%A3" class="mw-redirect" title="台灣">台灣</a>，21可以借指因成績不及格而被大學退學。</li> <li><a href="/wiki/%E8%8B%B1%E5%9B%BD" title="英国">英国</a><a href="/wiki/%E6%AD%8C%E6%89%8B" title="歌手">歌手</a><a href="/wiki/Adele" class="mw-redirect" title="Adele">Adele</a>的<a href="/wiki/%E4%B8%93%E8%BE%91" class="mw-redirect" title="专辑">专辑</a>：<a href="/wiki/21_(%E5%B0%88%E8%BC%AF)" class="mw-redirect" title="21 (專輯)">21 (专辑)</a>。</li> <li>马来西亚黑社会组织属洪门支派</li></ul> <div class="mw-heading mw-heading2"><h2 id="在其他領域中"><span id=".E5.9C.A8.E5.85.B6.E4.BB.96.E9.A0.98.E5.9F.9F.E4.B8.AD"></span>在其他領域中</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=21&action=edit&section=5" title="编辑章节：在其他領域中"><span>编辑</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/%E8%83%A1%E5%9C%8B%E8%88%88" title="胡國興">胡國興</a>於香港第五屆<a href="/wiki/2017%E5%B9%B4%E9%A6%99%E6%B8%AF%E7%89%B9%E5%88%A5%E8%A1%8C%E6%94%BF%E5%8D%80%E8%A1%8C%E6%94%BF%E9%95%B7%E5%AE%98%E9%81%B8%E8%88%89" title="2017年香港特別行政區行政長官選舉">特首選舉</a>所得的票數。</li> <li><a href="/wiki/%E4%BA%8C%E5%8D%81%E4%B8%80%E6%9D%A1" title="二十一条">二十一條</a>，日本向中國提出的條約</li></ul> <div class="mw-heading mw-heading2"><h2 id="参考文献"><span id=".E5.8F.82.E8.80.83.E6.96.87.E7.8C.AE"></span>参考文献</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=21&action=edit&section=6" title="编辑章节：参考文献"><span>编辑</span></a><span class="mw-editsection-bracket">]</span></span></div> <div class="reflist" style="list-style-type: decimal;"> <ol class="references"> <li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text"><cite class="citation web"><a rel="nofollow" class="external text" href="http://www.rsc.org/periodic-table">Royal Society of Chemistry - Visual Element Periodic Table</a>. <span class="reference-accessdate"> [<span class="nowrap">2013-01-31</span>]</span>. （原始内容<a rel="nofollow" class="external text" href="https://web.archive.org/web/20160410112333/http://www.rsc.org/periodic-table">存档</a>于2016-04-10）.</cite><span title="ctx_ver=Z39.88-2004&rfr_id=info%3Asid%2Fzh.wikipedia.org%3A21&rft.btitle=Royal+Society+of+Chemistry+-+Visual+Element+Periodic+Table&rft.genre=unknown&rft_id=http%3A%2F%2Fwww.rsc.org%2Fperiodic-table&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook" class="Z3988"><span style="display:none;"> </span></span></span> </li> <li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><cite class="citation web"><a href="/wiki/%E4%B8%AD%E8%8F%AF%E6%B0%91%E5%9C%8B%E6%95%99%E8%82%B2%E9%83%A8" title="中華民國教育部">中華民國教育部</a>. <a rel="nofollow" class="external text" href="https://dict.revised.moe.edu.tw/dictView.jsp?ID=144654&q=1&word=二十一">三七二十一</a>. 教育部重編國語辭典修訂本. <span class="reference-accessdate"> [<span class="nowrap">2025-03-05</span>]</span>.</cite><span title="ctx_ver=Z39.88-2004&rfr_id=info%3Asid%2Fzh.wikipedia.org%3A21&rft.atitle=%E4%B8%89%E4%B8%83%E4%BA%8C%E5%8D%81%E4%B8%80&rft.au=%E4%B8%AD%E8%8F%AF%E6%B0%91%E5%9C%8B%E6%95%99%E8%82%B2%E9%83%A8&rft.genre=unknown&rft.jtitle=%E6%95%99%E8%82%B2%E9%83%A8%E9%87%8D%E7%B7%A8%E5%9C%8B%E8%AA%9E%E8%BE%AD%E5%85%B8%E4%BF%AE%E8%A8%82%E6%9C%AC&rft_id=https%3A%2F%2Fdict.revised.moe.edu.tw%2FdictView.jsp%3FID%3D144654%26q%3D1%26word%3D%E4%BA%8C%E5%8D%81%E4%B8%80&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal" class="Z3988"><span style="display:none;"> </span></span></span> </li> </ol></div> <table class="metadata plainlinks stub" role="presentation" style="background:transparent"><tbody><tr class="noresize"><td><span typeof="mw:File"><a href="/wiki/File:Number_template.svg" class="mw-file-description"><img alt="小作品圖示" src="//upload.wikimedia.org/wikipedia/commons/thumb/5/59/Number_template.svg/23px-Number_template.svg.png" decoding="async" width="23" height="23" class="mw-file-element" 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