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$$\sum_{k=1}^{\infty} \Bigg[ \dfrac{(-1)^{k-1}}{k} \sum_{n=0}^{\infty} \dfrac{300}{k \cdot 2^n +5}\Bigg]= ?$$ I've never seen such a complex summation before. Can anyone help me? It would also be extremely helpful if anyone could tell me if they've seen this type of summation, and where to find similar problems. Has it appeared in any mathematics competitions, or are there similar questions for me to attempt?

Thanks in advance!

*note: This is Problem 29 from the Chen Jingrun's Cup Secondary School Mathematics Competition 2018. The answer is 137.

NoChance
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  • Welcome to MSE. This question is likely to be closed if you don't add more context. Where does the problem come form? Are you trying to evaluate the sum or to prove it converges? If the former, do you have any reason to be that exact evaluation is possible? Please respond by editing the question body. Many people browsing questions will vote to close without reading the comments. – saulspatz Mar 23 '19 at 13:12
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    @NoChance $n$ is the index of summation of the inner sum. – saulspatz Mar 23 '19 at 13:40
  • Surprising problem and result ! – Claude Leibovici Mar 23 '19 at 15:01
  • $$300\int_{0}^{1}\sum_{n\geq 0} x^4\log(1+x^{2^n}),dx =300\int_{0}^{1} -x^4\log(1-x),dx=\color{red}{137}.$$ – Jack D'Aurizio Mar 23 '19 at 20:27
  • @Jack D'Aurizio I'm sorry but I can't understand that ........Can you provide some topics regarding the solutions so that I can look it up and learn? Thanks! – Lee Laindingold Mar 24 '19 at 00:43
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    @NoChance It's $k\cdot 2^n + 5$ in the denominator, not $k(2^n + 5)$ – eyeballfrog Mar 24 '19 at 05:22
  • @eyeballfrog, thanks for your comment! – NoChance Mar 24 '19 at 05:30

1 Answers1

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This becomes easier by converting to an integral to get that weird linear term out of the denominator. Jack gave the quick version in the comments, but here's the more detailed explanation.

First, introduce an integral to convert from a zeta-like series to a power series: \begin{multline} \sum_{k=1}^\infty\frac{(-1)^{k-1}}{k}\sum_{n=0}^\infty \frac{300}{2^nk+5} = 300\sum_{k=1}^\infty\frac{(-1)^{k-1}}{k}\sum_{n=0}^\infty\int_0^1 x^{2^nk+4}dx \\= 300\int_0^1x^4\left[\sum_{n=0}^\infty\sum_{k=1}^\infty (-1)^{k-1}\frac{x^{2^nk}}{k}\right] dx = 300\int_0^1x^4\left[\sum_{n=0}^\infty\ln\left(1+x^{2^n}\right)\right] dx, \end{multline} where we used the series expansion $\ln(1+z) = \sum_{k=0}^\infty (-1)^{k-1}z^k/k$ to do the $k$ summation.

Next, evaluate the $n$ series using $$ \sum_{n=0}^\infty \ln(1+x^{2^n}) = \ln\left[\prod_{n=0}^\infty \left(1+x^{2^n}\right)\right] = \ln\left[\sum_{m=0}^\infty x^m\right] = \ln\left(\frac{1}{1-x}\right) = -\ln(1-x). $$ The identity with the infinite product can be seen by considering expanding each exponent $m$ as sums of powers of $2$. So now we've got our sum in terms of a nice integral, $$ \sum_{k=1}^\infty\frac{(-1)^{k-1}}{k}\sum_{n=0}^\infty \frac{300}{2^nk+5} = -300\int_0^1x^4\ln(1-x)dx $$ Last step is to use the identity $\int_0^1 x^n\ln(x)dx =-(n+1)^{-2}$ to get $$ -300\int_0^1x^4\ln(1-x)dx = -300\int_0^1(1-x)^4\ln(x)dx = 300\left(1 - \frac{4}{4} + \frac{6}{9}-\frac{4}{16}+\frac{1}{25}\right) = 137 $$ So, in conclusion, $$ \sum_{k=1}^\infty\frac{(-1)^{k-1}}{k}\sum_{n=0}^\infty \frac{300}{2^nk+5} =137 $$

eyeballfrog
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  • Thanks so much! It really helped! – Lee Laindingold Mar 24 '19 at 08:05
  • Actually,this question appeared in a secondary school maths competition so I doubt any calculus is involved..................but at least i found a way of solving it, thanks!It would also be helpful if anyone could provide a more secondary school solution haha – Lee Laindingold Mar 24 '19 at 08:10
  • @LeeLaindingold Infinite series are also calculus, and I believe they are usually taught after elementary integrals. – eyeballfrog Mar 24 '19 at 08:12