1.2. Relations and Functions
Definition 1.2.6: One-one, Onto, Bijection
A function f from A to B is called onto if for all b in B there is an a in A such that f(a) = b. Such functions are also called surjections.
A function f from A to B is called a bijection if it is one to one and onto, i.e. bijections are functions that are injective and surjective.