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A function f : X → Y is onto (or surjective) if given any y ∈ Y, ∃ x ∈ X such
that f (x) = y ®
A function f : X → Y is one-one and onto (or bijective), if f is both one-one
and onto ®
Given a finite set X, a function f : X → X is one-one (respectively onto) if and
only if f is onto (respectively one-one) This is the characteristic property of a
finite set