Infinite Poly-summation

Calculus Level 5

S = k = 1 ( 1 k 2 ψ ( 1 ) ( k + 1 2 ) ) \large S = \sum_{k=1}^\infty \left ( \dfrac1{k^2} {\psi^{(1)} \left( \frac{k+1}2\right) } \right)

Let ψ ( 1 ) ( ) \psi^{(1)} (\cdot) denote the Trigamma function , ψ ( 1 ) ( z ) = n = 0 1 ( z + n ) 2 \displaystyle \psi^{(1)}(z) = \sum_{n=0}^\infty \dfrac1{(z+n)^2} .

If S S can be expressed as π A B \dfrac{\pi^A}B , where A A and B B are integers, find A + B A+B .


The answer is 52.

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1 solution

Mark Hennings
May 11, 2017

S = k = 1 1 k 2 ψ ( 1 ) ( k + 1 2 ) = k = 1 1 k 2 0 t 1 e t e 1 2 ( k + 1 ) t d t = 0 t e 1 2 t 1 e t L i 2 ( e 1 2 t ) d t = 4 0 1 ln u L i 2 ( u ) 1 u 2 d u \begin{array}{c}S & = \displaystyle \sum_{k=1}^\infty \frac{1}{k^2} \psi^{(1)}\big(\tfrac{k+1}{2}\big) \; = \; \sum_{k=1}^\infty \frac{1}{k^2} \int_0^\infty \frac{t}{1-e^{-t}} e^{-\frac12(k+1)t}\,dt \\ & = \displaystyle \int_0^\infty \frac{te^{-\frac12t}}{1 - e^{-t}} \mathrm{Li}_2\big(e^{-\frac12t}\big)\,dt \; = \; -4\int_0^1 \frac{\ln u\, \mathrm{Li}_2(u)}{1-u^2}\,du \end{array} Since L i 2 ( u ) 1 u = n = 1 H n ( 2 ) u n u < 1 \frac{\mathrm{Li}_2(u)}{1-u} \; = \; \sum_{n=1}^\infty H^{(2)}_n u^n \hspace{2cm} |u| < 1 we deduce that 0 1 L i 2 ( u ) ln u 1 u d u = n = 1 H n ( 2 ) ( n + 1 ) 2 = ζ ( 2 , 2 : 1 , 1 ) = 1 120 π 4 \int_0^1 \frac{\mathrm{Li}_2(u)\,\ln u}{1-u}\,du \; = \; -\sum_{n=1}^\infty \frac{H^{(2)}_n}{(n+1)^2} \; = \; -\zeta(2,2:1,1) \; = \; -\tfrac{1}{120}\pi^4 On the other hand L i 2 ( u ) 1 + u = ( j 0 u j ) ( k 1 u k k 2 ) = N = 1 ( 1 ) N ( k = 1 N ( 1 ) k k 2 ) u N \frac{\mathrm{Li}_2(u)}{1+u} \; = \; \left(\sum_{j \ge 0}u^j\right)\left(\sum_{k \ge 1} \frac{u^k}{k^2}\right) \; = \; \sum_{N=1}^\infty (-1)^N\left(\sum_{k=1}^N \frac{(-1)^k}{k^2}\right)u^N and hence 0 1 L i 2 ( u ) ln u 1 + u d u = N 1 ( 1 ) N ( N + 1 ) 2 ( k = 1 N ( 1 ) k k 2 ) = ζ ( 2 , 2 : 1 , 1 ) = 1 480 π 4 \int_0^1 \frac{\mathrm{Li}_2(u)\,\ln u}{1+u}\,du \; = \; -\sum_{N \ge 1} \frac{(-1)^N}{(N+1)^2}\left(\sum_{k=1}^N \frac{(-1)^k}{k^2}\right) \; = \; -\zeta(2,2:-1,-1) \; = \;-\tfrac{1}{480}\pi^4 and hence S = 2 ( 0 1 L i 2 ( u ) ln u 1 u d u + 0 1 L i 2 ( u ) ln u 1 + u d u ) = 1 48 π 4 S \; = \; -2\left( \int_0^1 \frac{\mathrm{Li}_2(u)\,\ln u}{1-u}\,du + \int_0^1 \frac{\mathrm{Li}_2(u)\,\ln u}{1+u}\,du \right) \; = \; \tfrac{1}{48}\pi^4 making the answer 4 + 48 = 52 4 + 48 = \boxed{52} .

@Mark Hennings , we really liked your comment, and have converted it into a solution.

Brilliant Mathematics Staff - 4 years, 1 month ago

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.)

Mat Met - 4 years, 1 month ago

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