Let A and B be two sets. Then a relation R from A to B is a subset of A×B.
R is a relation from A to B => R is a subset of A×B.
Total number of relations: If A and B are two non empty sets with m and n elements respectively, A×B consists of mn ordered pairs.
Since each subset defines a relation from A to B, so total number of relations from A to B is 2mn.
2.3 Types of relations
2.4 (2.5 in the book) Composition of Relations