The number of onto functions from $A \rightarrow B$ when $a \ge b$ is:
$\sum_{j=0}^{b}$$(-1^{j})$ ${b}\choose{j}$$(b-j)^{a}$
I cannot find a derivation for this. Can someone please help?
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1check https://math.stackexchange.com/questions/500674/number-of-surjective-functions-from-a-to-b – janmarqz Apr 11 '20 at 02:23
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I am looking for the derivation. – Apr 11 '20 at 04:00
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@User10101.. isn't it there https://math.stackexchange.com/questions/500674/number-of-surjective-functions-from-a-to-b/1302618#1302618 ? – janmarqz Apr 11 '20 at 05:13