Assertion: set's number | /r/HasProperty | defined in zermelo–fraenkel zfset theory
Facets:
value: recursively
type: MANNER
value: by letting 0
type: MANNER
Source sentences:
text: In Zermelo–Fraenkel (ZF) set theory, the natural numbers are defined recursively by letting 0 = {} be the empty set and n + 1 = n ∪ {n} for each n.