S = k = 1 ∑ ∞ ( k 2 1 ψ ( 1 ) ( 2 k + 1 ) )
Let ψ ( 1 ) ( ⋅ ) denote the Trigamma function , ψ ( 1 ) ( z ) = n = 0 ∑ ∞ ( z + n ) 2 1 .
If S can be expressed as B π A , where A and B are integers, find A + B .
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@Mark Hennings , we really liked your comment, and have converted it into a solution.
Hi Mark Hennings, I appreciated a lot your last comment under the report section, but I couldn't answer any sooner. Gratefully, they updated the problem and fixed my mistake (which I'm sorry for), exactly as you announced.
This extended solution appears wonderful. (Hopefully one day I will be able to read it.)
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S = k = 1 ∑ ∞ k 2 1 ψ ( 1 ) ( 2 k + 1 ) = k = 1 ∑ ∞ k 2 1 ∫ 0 ∞ 1 − e − t t e − 2 1 ( k + 1 ) t d t = ∫ 0 ∞ 1 − e − t t e − 2 1 t L i 2 ( e − 2 1 t ) d t = − 4 ∫ 0 1 1 − u 2 ln u L i 2 ( u ) d u Since 1 − u L i 2 ( u ) = n = 1 ∑ ∞ H n ( 2 ) u n ∣ u ∣ < 1 we deduce that ∫ 0 1 1 − u L i 2 ( u ) ln u d u = − n = 1 ∑ ∞ ( n + 1 ) 2 H n ( 2 ) = − ζ ( 2 , 2 : 1 , 1 ) = − 1 2 0 1 π 4 On the other hand 1 + u L i 2 ( u ) = ( j ≥ 0 ∑ u j ) ( k ≥ 1 ∑ k 2 u k ) = N = 1 ∑ ∞ ( − 1 ) N ( k = 1 ∑ N k 2 ( − 1 ) k ) u N and hence ∫ 0 1 1 + u L i 2 ( u ) ln u d u = − N ≥ 1 ∑ ( N + 1 ) 2 ( − 1 ) N ( k = 1 ∑ N k 2 ( − 1 ) k ) = − ζ ( 2 , 2 : − 1 , − 1 ) = − 4 8 0 1 π 4 and hence S = − 2 ( ∫ 0 1 1 − u L i 2 ( u ) ln u d u + ∫ 0 1 1 + u L i 2 ( u ) ln u d u ) = 4 8 1 π 4 making the answer 4 + 4 8 = 5 2 .