f(n,m,k)=∑i=1n(i+m−2m−1)(n−i+kk)=(n+m+k−1n−1)\begin{aligned}f(n,m,k)&=\sum_{i=1}^n\binom{i+m-2}{m-1}\binom{n-i+k}k\\&=\binom{n+m+k-1}{n-1}\end{aligned}f(n,m,k)=i=1∑n(m−1i+m−2)(kn−i+k)=(n−1n+m+k−1)
求大佬看看怎么推的
