Tuesday, 25 April 2017

calculus - Simplification of a nested sum



I have a nested sum like so:
kk1=k0 ...kkn=kn1n times1 with  n,k0,kN, kk0



Is there a general, shorter representation that spares me calculating the actual sums?


Answer




\displaystyle\sum_{k_0\le k_1\le\cdots\le k_n\le k^*}1=\sum_{1\le k_1-k_0+1< k_2-k_0+2<\cdots< k_n-k_0+n\le k^*-k_0+n}1={k^*-k_0+n\choose n}


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