You get to a pont where a certain theorem is bound to be proved, independent of any particular individual, because it is just in the path of development. This section will introduce some frequently used terms. Let S and T be finite sets. In general, no. Equivalently, a function f from S to T is surjective if each t in T is the image of some s in S under f.
T is surjective if and only if it is injective.
Both f1 and f2 are bijections. Let S be a set. This is a very general notion. There are lots and lots of relations in mathematics - inequality symbols, functions, subset symbols are all common examples of relations. Note that R is a relation.