Surjections 网n四=-*(四) m kn k=1 (f-1(0),f-1(1),.,f-1(m-1) ordered m-partition of [n] 例-m{h} {}=-() knSurjections = ⇤ m k=1 (1)mk m k ⇥ kn [n] onto ⇥ [m] (f 1(0), f 1(1),...,f 1(m 1)) ordered m-partition of [n] [n] onto [m] = m! n m n m = 1 m! m k=1 (1)mk m k kn