1(nm)∑k=mn−1(k−1m−1)n−k=mn∑k=mn−11k\frac 1 {\binom n m} \sum_{k=m}^{n-1} \frac{\binom{k-1}{m-1}}{n-k} =\frac m n \sum_{k=m}^{n-1}\frac 1 k(mn)1∑k=mn−1n−k(m−1k−1)=nm∑k=mn−1k1
