（英語版） https://en.wikipedia.org/wiki/Zermelo_set_theory Zermelo set theory AXIOM VII. Axiom of infinity (Axiom des Unendlichen) "There exists in the domain at least one set Z that contains the null set as an element and is so constituted that to each of its elements a there corresponds a further element of the form {a}, in other words, that with each of its elements a it also contains the corresponding set {a} as element." (引用終り) 以上
