2楼: Originally posted by hank612 at 2014-05-18 08:53:40
首先，如果正整数 i_1,...,i_k 满足 和为n, 那么这种组合的个数为 {(n-k)+(k-1) \choose (k-1)}={n-1 \choose k-1}

http://math.stackexchange.com/questions/217597/number-of-ways-to-write-n-as-a-sum-of-k- ...