sm (+ (k-1) k21 2 k！ k！ If k is fixed, pk(n) nk-1 asm→ X k!(k-1)月￾n￾1 k￾1 ⇥ k! ￾ pk(n) ￾ ￾n+ k(k￾1) 2 ￾1 k￾1 ⇥ k! pk(n) ⇥ nk￾1 k!(k ￾ 1)! as n ￾ ⇥ If k is fixed