How can I prove that the number of surjective mapping from the set $A$ to a set $B$ is $\sum\limits_{r=1}^n(-1)^{n-r} \binom{ n}{ r}r^m $, where $|A|=m$ and $|B|=n$.
I can't get any idea how to prove it. Is it possible to prove it?
2026-05-06 02:16:39.1778033799
General Formula for Number of Surjective mappings from the set $A$ to a set $B$.
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