Prove $\int_0^1\frac{1-x}{(\ln x)(1+x)}\ dx=\ln\left(\frac2{\pi}\right)$

calculusdefinite integralsintegrationreal-analysissequences-and-series

A friend asked me to prove

$$\int_0^1\frac{1-x}{(\ln x)(1+x)}\ dx=\ln\left(\frac2{\pi}\right)$$

and here is my approach:

\begin{align}
I&=\int_0^1\frac{1-x}{\ln x}\frac1{1+x}\ dx\\
&=\int_0^1\left(-\int_0^1x^y\ dy\right)\frac1{1+x}\ dx\\
&=\int_0^1\left(-\int_0^1\frac{x^y}{1+x}\ dx\right)\ dy\\
&=\int_0^1\left((-1)^n\sum_{n=1}^\infty\int_0^1x^{y+n-1}\ dx\right)\ dy\\
&=\int_0^1\left(\sum_{n=1}^\infty\frac{(-1)^n}{y+n}\right)\ dy\\
&=\sum_{n=1}^\infty(-1)^n\int_0^1\frac1{y+n}\ dy\\
&=\sum_{n=1}^\infty(-1)^n\left[\ln(n+1)-\ln(n)\right]\tag{1}
\end{align}


Now how can we finish this alternating sum into $\ln\left(\frac2{\pi}\right)$?

My idea was to use

$$\operatorname{Li}_a(-1)=(1-2^{-a})\zeta(a)=\sum_{n=1}^\infty\frac{(-1)^n}{n^a}$$

and if we differentiate both sides with respect to $a$ we get

$$2^{1-a}(\zeta^{'}(a)-\ln2\zeta(a))-\zeta^{'}(a)=\sum_{n=1}^\infty\frac{(-1)^{n-1}\ln(n)}{n^a}\tag{2}$$

wolfram says that $\sum_{n=1}^\infty(-1)^n\ln(n)$ is divergent which means we can not take the limit for (2) where $a\mapsto 0$ which means we can not break the sum in (1) into two sums. So any idea how to do (1)?


Other question is, I tried to simplify the sum in (1) as follows

\begin{align}
S&=\sum_{n=1}^\infty(-1)^n\left[\ln(n+1)-\ln(n)\right]\\
&=-\left[\ln(2)-\ln(1)\right]+\left[\ln(3)-\ln(2)\right]-\left[\ln(4)-\ln(3)\right]+\left[\ln(5)-\ln(4)\right]-…\\
&=-2\ln(2)+2\ln(3)-2\ln(4)+…\\
&=2\sum_{n=1}^\infty(-1)^n\ln(n+1)
\end{align}

which is divergent again. what went wrong in my steps?

Thanks.

Best Answer

We've $\displaystyle \sum_{n \ge 1} (-1)^n \left[\log(n+1)- \log(n)\right] = \sum_{n \ge 1}(-1)^n\log\left(\frac{n+1}{n}\right) = \log(\mathcal{P})$ where:

$\displaystyle \mathcal{P} = \prod_{n \ge 1}\bigg(\frac{n+1}{n}\bigg)^{(-1)^n} = \prod_{n \ge 1} \bigg(\frac{2n+1}{2n}\bigg) \bigg(\frac{2n}{2n-1}\bigg)^{-1} = \prod_{n \ge 1} \bigg(\frac{2n+1}{2n}\cdot \frac{2n-1}{2n}\bigg) = \frac{2}{\pi}. $

Where the last step is the Wallis product for $\pi$.