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Full Idea
A function 'maps A onto B' if the domain of relating things is set A, and the things related to are set B.
Gist of Idea
A function 'maps A onto B' if the relating things are set A, and the things related to are set B
Source
Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0)
Book Ref
Enderton,Herbert B.: 'A Mathematical Introduction to Logic' [Academic Press 2001], p.5