Zermelo–Fraenkel set theory
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In set theory, Zermelo–Fraenkel set theory, named after mathematicians Ernst Zermelo and Abraham Fraenkel, is an axiomatic system that was proposed in the early twentieth century in order to formulate
Ernst Zermelo (1871–1953)
In the foundations of mathematics, von Neumann–Bernays–Gödel set theory is an axiomatic set theory that is a conservative extension of Zermelo–Fraenkel–choice set theory. NBG introduces the notion of
Zermelo–Fraenkel set theory
…does not allow for the existence of a universal set (a set containing all sets) nor for unrestricted comprehension, thereby avoiding Russell's paradox. Von Neumann–Bernays–Gödel set theory (NBG) is a commonly used conservative extension of Zermelo–Fraenkel set theory that does allow explicit treatment of proper classes. There are many…
Kurt Gödel, c. 1926
Paul Bernays