@[hanzhongtlx](/user/184500) md 我要重打一遍了
by EndSaH @ 2020-07-05 21:47:09
@[hanzhongtlx](/user/184500) orz%%%wtcl看不懂
by Laser_Crystal @ 2020-07-05 21:47:49
@[EndSaH](/user/91252) 谢谢谢谢/se
by hanzhongtlx @ 2020-07-05 21:48:10
@[Lattle_Car](/user/231169) fake
by hanzhongtlx @ 2020-07-05 21:48:18
@[EndSaH](/user/91252) 加几个$不就行了？
by Cheng、熙® @ 2020-07-05 21:50:54
@[Cheng、熙®](/user/125359) 你可以试试...
by 血色黄昏 @ 2020-07-05 21:51:24
by EndSaH @ 2020-07-05 21:53:06
$ \begin{aligned} k! &= \sum {i = 0} ^k (-1) ^i {k \choose i} (m - i) ^k \ &= \sum {j = 0} ^k S2(k, j) j! \sum _{i = 0} ^k {m - i \choose j} (-1) ^i {k \choose i} \end{aligned} $
by Cheng、熙® @ 2020-07-05 21:53:50
@[hanzhongtlx](/user/184500) 因为有个 $x ^k$ 在里面，所以当 $j = k$ 时这个式子取 $1$，否则取 $0$
by EndSaH @ 2020-07-05 21:53:56
@[血色黄昏](/user/220426) 我孤陋寡闻了。。
by Cheng、熙® @ 2020-07-05 21:54:45