# Thread: need help on onto function...

1. ## need help on onto function...

Let A ={1,2,3,...,m} and B={1,2,3,...,n}, where m>n. Find an expression for the number of function f : A--->B that are onto. Prove that the expression is correct.

2. $\text{Surj}(m,n)=\sum\limits_{k=0}^{n} {(-1)^k \binom{n}{k}(n-k)^m}$