Surjections 网mm=-1m(g〉 m k=1 (f-1(0),f-1(1),.,f-1(m-1) ordered partition of [m] n,lonlm {}=-(） m knSurjections = ⇤ m k=1 (￾1)m￾k ￾m k ⇥ kn ￾ ￾ ￾[n] onto ￾￾￾⇥ [m] ￾ ￾ ￾ (f ￾1(0), f ￾1(1),...,f ￾1(m ￾ 1)) ordered partition of [m] ￾ ￾ ￾[n] onto ￾￾￾￾ [m] ￾ ￾ ￾ = m! ￾ n m ￾ ￾ n m ￾ = 1 m! ￾ m k=1 (￾1)m￾k ￾m k ￾ kn