组合恒等式:1.\(n\choosem\)=\(n-1\choosem\)+\(n-1\choosem-1\)2.下降幂\(n^{m}\)就是\(A^{m}_{n}\)3.\(\sum^{m}_{i=0}{i\choosen}={m+1\choosen+1}\)4.范德蒙德卷积\(\sum^{k}_{i=0}{n\choosei}{m\choosek-i}={n+m\choosek}\)5.\(\sum_{i