Class

Class function

A class function¹ is a generalization of a set function, such that the domain may be any Class set Exactly how this is treated depends on the axiomatic set theory employed, but generally one takes a Relation class with the restriction that each element in the domain is related to exactly one element in the codomain. Thus in NBG

while in ZF we may treat the notion indirectly by a predicative formula such that

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Footnotes

1. Not to be confused with a group class function, which is often referred to by the same name. ↩