摘要：
$$\begin &\sum_^n\sum_i \begini\j\end2j j! \ =&\sum_^n\sum_n \begini\j\end2j j! \ =& \sum_^n \sum_n 2jj! \frac{1}{j!} \sum_^j (-1)^k \binom (j-k)^i \ 阅读全文
摘要：
$$\begin &\sum_n f(k) xk \binom \ =&\sum_m a_i\sum_n ki xk \binom \ =&\sum_^m a_i\sum_nxk \binom \sum_^i \binom \begini \ j\endj! \ =&\sum_m a_i \sum_ 阅读全文