Question

Let A and B be nonempty finite sets with |A| = m and |B| = n
If m > n, then find the number of onto functions from A to B

Answer 1

HI!!

Answer 2

you need the formula or an explanation?

Answer 3

if you just need the formula it is \[\sum_{k=0}^n(-1)^k\binom{n}
{k}(n-k)^m\]

Answer 4

if you need an explanation, here is a decent one
http://math.stackexchange.com/questions/334420/number-of-onto-functions