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