Surjections of f (nl onto,(ml U=[m→[ml A=[ml→([ml\{i}) U-UA,=∑(-1)川1A icml】 E[m] AI=∩A: Ao=U 2∈ISurjections f : [n] onto ￾￾￾⇥ [m] # of U = [n] ￾ [m] Ai = [n] ￾ ([m] \ {i}) AI = ￾ i￾I Ai A￾ = U ￾ ￾ ￾ ￾ ￾ ￾ U ￾ ⇥ i￾[m] Ai ￾ ￾ ￾ ￾ ￾ ￾ = ￾ I￾[m] (￾1)|I| |AI |