(n+1)k+1−1=∑i=0k(k+1i)∑j=1nji(n+1)^{k+1}-1=\sum\limits_{i=0}^k\binom{k+1}i\sum\limits_{j=1}^nj^i(n+1)k+1−1=i=0∑k(ik+1)j=1∑nji
