Cute looking nasty summation!

Calculus Level 5

n = 1 ( H n ) 3 n 4 = A B ζ ( C ) D E ζ ( F ) ζ ( G ) + H ζ ( I ) ζ ( J ) \sum _{ n=1 }^{ \infty }{ \frac { { \left( { H }_{ n } \right) }^{ 3 } }{ { n }^{ 4 } } } =\frac { A }{ B } \zeta \left( C \right) -\frac { D }{ E } \zeta \left( F \right) \zeta \left( G \right) +H\zeta \left( I \right) \zeta \left( J \right)

Here, A , B , C , D , E , F , G , H , I , J A,B,C,D,E,F,G,H,I,J are positive integers.

Find: min { A + B + C + D + E + F + G + H + I + J } \min { \left\{ A+B+C+D+E+F+G+H+I+J \right\} }


The answer is 325.

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1 solution

Mark Hennings
Mar 13, 2016

Note that k = 1 H k 3 k 4 = 1 + k = 1 H k + 1 3 ( k + 1 ) 4 = 1 + k = 1 1 ( k + 1 ) 4 [ H k + 1 k + 1 ] 3 = 1 + k = 1 ( H k 3 ( k + 1 ) 4 + 3 H k 2 ( k + 1 ) 5 + 3 H k ( k + 1 ) 6 + 1 ( k + 1 ) 7 ) = s h ( 3 , 4 ) + 3 s h ( 2 , 5 ) + 3 s h ( 1 , 6 ) + ζ ( 7 ) \begin{array}{rcl} \displaystyle \sum_{k=1}^\infty \frac{H_k^3}{k^4} & = & \displaystyle 1 + \sum_{k=1}^\infty \frac{H_{k+1}^3}{(k+1)^4} \; = \; 1 + \sum_{k=1}^\infty \frac{1}{(k+1)^4}\big[H_k + \tfrac{1}{k+1}\big]^3 \\ & = & \displaystyle 1 + \sum_{k=1}^\infty \left(\frac{H_k^3}{(k+1)^4} + 3\frac{H_k^2}{(k+1)^5} + 3\frac{H_k}{(k+1)^6} + \frac{1}{(k+1)^7}\right) \\ & = & s_h(3,4) + 3s_h(2,5) + 3s_h(1,6) + \zeta(7) \end{array} where we are using the Euler sums s h ( m , n ) = k = 1 H k m ( k + 1 ) n , s_h(m,n) \; = \; \sum_{k=1}^\infty \frac{H_k^m}{(k+1)^n} \;, it can be shown by standard results concerning Euler sums (formulae for s h ( 1 , n ) s_h(1,n) , s h ( 2 , 2 n 1 ) s_h(2,2n-1) and s h ( 3 , 4 ) s_h(3,4) are all known in terms of Zeta functions) that k = 1 H k 3 k 4 = 231 16 ζ ( 7 ) 51 4 ζ ( 3 ) ζ ( 4 ) + 2 ζ ( 2 ) ζ ( 5 ) , \sum_{k=1}^\infty \frac{H_k^3}{k^4} \; =\; \tfrac{231}{16}\zeta(7) - \tfrac{51}{4}\zeta(3)\zeta(4) + 2\zeta(2)\zeta(5) \;, making the answer 231 + 16 + 7 + 51 + 4 + 3 + 4 + 2 + 2 + 5 = 325 231 + 16 + 7 + 51 + 4 +3 + 4 + 2 + 2 + 5 \,=\, \boxed{325} .

My method was similar to what I posted here . I used quasi-shuffle identity to get it to multi-harmonic numbers.

Aditya Kumar - 5 years, 3 months ago

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