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Math Help - need help on onto function...

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