If number of elements in sets A and B are  and m, n respectively,then the number of relations from A to B is

                                                                                   $$\begin{gathered} No. of order pairs from A to\hspace{0.17em}B is n\left(A\right)n\left(B\right) \\ Number of relations from A to B is{2}^{n\left(A\right)n\left(B\right)}= 2^{mn} \end{gathered}$$