n = 1 ∑ ∞ n 4 ( H n ) 3 = B A ζ ( C ) − E D ζ ( F ) ζ ( G ) + H ζ ( I ) ζ ( J )
Here, 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 }
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My method was similar to what I posted here . I used quasi-shuffle identity to get it to multi-harmonic numbers.
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Note that k = 1 ∑ ∞ k 4 H k 3 = = = 1 + k = 1 ∑ ∞ ( k + 1 ) 4 H k + 1 3 = 1 + k = 1 ∑ ∞ ( k + 1 ) 4 1 [ H k + k + 1 1 ] 3 1 + k = 1 ∑ ∞ ( ( k + 1 ) 4 H k 3 + 3 ( k + 1 ) 5 H k 2 + 3 ( k + 1 ) 6 H k + ( k + 1 ) 7 1 ) s h ( 3 , 4 ) + 3 s h ( 2 , 5 ) + 3 s h ( 1 , 6 ) + ζ ( 7 ) where we are using the Euler sums s h ( m , n ) = k = 1 ∑ ∞ ( k + 1 ) n H k m , it can be shown by standard results concerning Euler sums (formulae for s h ( 1 , n ) , s h ( 2 , 2 n − 1 ) and s h ( 3 , 4 ) are all known in terms of Zeta functions) that k = 1 ∑ ∞ k 4 H k 3 = 1 6 2 3 1 ζ ( 7 ) − 4 5 1 ζ ( 3 ) ζ ( 4 ) + 2 ζ ( 2 ) ζ ( 5 ) , making the answer 2 3 1 + 1 6 + 7 + 5 1 + 4 + 3 + 4 + 2 + 2 + 5 = 3 2 5 .