Surjections U=[m]→[m A:=[m→(m\{i) Ao=U Ar=∩Ai=[m→(Im\I) i∈I |Ar=(m-1I)” ∩A, =>(-1)|A iElm] IC[m] ∑(-1(m-1W”=-1()m- = IC[m] =-m-(）Surjections U = [n] ￾ [m] Ai = [n] ￾ ([m] \ {i}) AI = ￾ i￾I A￾ = U Ai = [n] ￾ ([m] \ I) |AI | = (m ￾ |I|) n = ⇤ m k=1 (￾1)m￾k ￾m k ⇥ kn = ￾ I￾[m] (￾1)|I| (m ￾ |I|) n ￾ I￾[m] (￾1)|I| |AI | ￾ ￾ ￾ ￾ ￾ ￾ ￾ i￾[m] Ai ￾ ￾ ￾ ￾ ￾ ￾ = = ￾ m k=0 (￾1)k ￾m k ￾ (m ￾ k) n