1fa a b C and B 1 3 1 0 1 3 then the number of injections that can be defined from A to B is

Definition: Relation

A relation from a set \[A\] to a set \[B\] is a subset of \[A \times B\]. Hence, a relation \[R\] consists of ordered pairs \[[a,b]\], where \[a\in A\] and \[b\in B\]. If \[[a,b]\in R\], we say that is related to , and we also write \[a\,R\,b\].

Remark

We can also replace \[R\] by a symbol, especially when one is readily available. This is exactly what we do in, for example, \[a