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It's my first post here and I was wondering if someone could help me with evaluating the definite integral $$ \int_0^{\Large\frac{\pi}{4}} \log\left( \cos x\right) \, \mathrm{d}x $$ Thanks in advance, any help would be appreciated.

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What did you try ? maybe putting $t=\cos(x)$ would help – Belgi Sep 19 '12 at 12:12
There is an answer, but I cannot say how it is found:[Log[Cos[x]]%2C{x%2C0%2CPi%2F4}] – Siminore Sep 19 '12 at 12:16
@Siminore: That link is broken; here's one that works. – joriki Sep 19 '12 at 12:20
@Souvik : You mean 'evaluating'. – mick Sep 19 '12 at 13:12

Write $$\log(\cos(x))=\log\left(\frac12 e^{ix}(1+e^{-2ix})\right)\\ =-\log 2 + ix +\sum_{k=1}^\infty\frac{(-1)^{k+1}}{k}e^{-2ikx}.$$ Then integrate term by term to obtain $$\int_0^{\pi/4}\log(\cos(x))dx=-\frac{\pi}{4}\log 2 +i\frac{\pi^2}{32}+\frac{i}{2}\sum_{k=1}^\infty\frac{(-1)^{k+1}}{k^2}\left[e^{-ik\pi/2}-1\right].$$ The odd terms of the series with $e^{-ik\pi/2}$ give rise to the Catalan constant, and the even terms combine with the other infinite series to cancel the $i\pi^2/32$ term.

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Let $$ I=\int_0^{\Large\frac\pi4}\ln(\sin x)\ dx\qquad\text{and}\qquad J=\int_0^{\Large\frac\pi4}\ln(\cos x)\ dx $$ then \begin{align} I+J&=\int_0^{\Large\frac\pi4}\ln(\sin x\cos x)\ dx\\ &=\int_0^{\Large\frac\pi4}\ln\left(\frac12\sin 2x\right)\ dx\\ &=\int_0^{\Large\frac\pi4}\ln(\sin 2x)\ dx-\int_0^{\Large\frac\pi4}\ln2\ dx\\ &=\frac12\int_0^{\Large\frac\pi2}\ln(\sin y)\ dy-\frac\pi4\ln2\qquad\color{red}{\Rightarrow}\qquad \text{set}\ y=2x\\ &=-\frac\pi2\ln2 \end{align} and \begin{align} I-J&=\int_0^{\Large\frac\pi4}\ln\left(\frac{\sin x}{\cos x}\right)\ dx\\ &=\int_0^{\Large\frac\pi4}\ln\left(\tan x\right)\ dx\\ &=\int_0^{1}\frac{\ln t}{1+t^2}\ dt\qquad\color{red}{\Rightarrow}\qquad \text{set}\ t=\tan x\\ &=\int_0^{1}\sum_{n=1}^\infty(-1)^n t^{2n}\ln t\ dt\\ &=\sum_{n=1}^\infty(-1)^n\int_0^{1} t^{2n}\ln t\ dt\\ &=-\sum_{n=1}^\infty\frac{(-1)^n}{(2n+1)^2}\\ &=-G, \end{align} where $G$ is Catalan's constant. Therefore $$ I=\int_0^{\Large\frac\pi4}\ln(\sin x)\ dx=-\frac12\left(G+\frac\pi2\ln2\right) $$ and $$ J=\int_0^{\Large\frac\pi4}\ln(\cos x)\ dx=\frac12\left(G-\frac\pi2\ln2\right). $$

References :

$[1]\ \ \displaystyle\int_0^{\Large\frac\pi2}\ln(\sin y)\ dy=\int_0^{\Large\frac\pi2}\ln(\cos y)\ dy=-\frac\pi2\ln2$

$[2]\ \ \displaystyle\int_0^1 x^\alpha \ln^k x\ dx=\frac{(-1)^k k!}{(\alpha+1)^{k+1}}, \qquad\text{for }\ k=0,1,2,\ldots$

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The $n$-index in the Catalan constant must start at $\ \color{#c00000}{0}\ $ instead of 1. You can check it at this link. – Felix Marin Dec 28 '14 at 6:03

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\begin{align}&\color{#66f}{\large\int_{0}^{\pi/4}\ln\pars{\cos\pars{x}}\,\dd x} = \int_{-\pi/2}^{-\pi/4}\ln\pars{-\sin\pars{x}}\,\dd x = \int_{\pi/4}^{\pi/2}\ln\pars{\sin\pars{x}}\,\dd x \\[5mm]&=\int_{\pi/4}^{\pi/2}\overbrace{\bracks{% -\ln\pars{2} - \sum_{k\ =\ 1}^{\infty}{\cos\pars{2kx} \over k}}}^{\dsc{\ln\pars{\sin\pars{x}}}}\,\dd x =-\,{1 \over 4}\,\pi\ln\pars{2} -\sum_{k\ =\ 1}^{\infty}{1 \over k}\int_{\pi/4}^{\pi/2}\cos\pars{2kx}\,\dd x \\[5mm]&=-\,{1 \over 4}\,\pi\ln\pars{2} -\sum_{k\ =\ 1}^{\infty}{1 \over k}{\sin\pars{k\pi} - \sin\pars{k\pi/2} \over 2k} =-\,{1 \over 4}\,\pi\ln\pars{2} +\half\sum_{k\ =\ 1}^{\infty}{\sin\pars{k\pi/2} \over k^{2}} \\[5mm]&=-\,{1 \over 4}\,\pi\ln\pars{2} +\half\sum_{k\ =\ 0}^{\infty}{\sin\pars{k\pi + \pi/2} \over \pars{2k + 1}^{2}} =-\,{1 \over 4}\,\pi\ln\pars{2} +\half\ \underbrace{\sum_{k\ =\ 0}^{\infty}{\pars{-1}^{k} \over \pars{2k + 1}^{2}}} _{\ds{\mbox{Catalan Constant}\ \dsc{G}}} \\[5mm]&=\color{#66f}{\large-\,{1 \over 4}\,\pi\ln\pars{2} + \half\,G} \end{align}

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The integral: $$S=\int_0^\frac{\pi}{4}\log(\cos(x))dx=\frac{1}{4}(2C-\pi \log 2)$$ where $C$ is the Catalan constant.

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So Wolfram|Alpha tells us. Can you offer any insight into how this result is obtained? – joriki Sep 19 '12 at 12:22
Hmhm, I saw that answer with Catalan constant,can there be a more elementary solution without infinite series? – Souvik Sep 19 '12 at 12:26
Use $\cos(x)=\frac{\exp(i\theta)+\exp(-i\theta)}{2}$ and then integrate it. – Riccardo.Alestra Sep 19 '12 at 12:33
The reason the Catalan constant has a special name is that it cannot (as far as we know) be written in terms of more familiar things. – GEdgar Sep 19 '12 at 13:07
@ Riccardo: This helps us to find the part with $\pi log(2)$ but how do you find the Catalan constant with that ? – mick Sep 19 '12 at 13:16

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