If no. of elements in set A = m, and no. of elements in set B = n.
Then, If , No. of onto functions from set A to set B =
For eg. if mapping is done such that there are 5 balls to be put in 5 boxes, then no. of ways of putting, such that no box remains empty (No. of onto functions) =
If then no. of onto functions = 0
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