到達不能基数
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Sierpiński, Wacław; Tarski, Alfred (1930), “Sur une propriété caractéristique des nombres inaccessibles”, Fundamenta Mathematicae 15: 292–300, ISSN 0016-2736, http://matwbn.icm.edu.pl/tresc.php?wyd=1&tom=15
Zermelo, Ernst (1930), “Über Grenzzablen und Mengenbereiche”, Fundamenta Mathematicae 16: 29–47, ISSN 0016-2736, http://matwbn.icm.edu.pl/tresc.php?wyd=1&tom=16