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I was trying to evaluate $$\int^1_0 \frac{\log(1+x)}{x}dx.$$

I expanded $\log(1+x) $ as $x -\frac{x^2}{2}... $ and got the answer. I would like to know if there is any way to do it without series expanding.

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5  
You then get another integral, which isn't easy to solve. –  Ishan Banerjee Jan 28 '13 at 10:28
2  
$$ \int_0^1 \frac{\log(1+x)}{x} dx =\frac{\pi^2}{12} $$ –  Rustyn Jan 28 '13 at 10:40
    
@Rustyn Yazdanpour I also got the same answer.I solved it in terms of $\zeta (2)$. But as I want to avoid series expansion or using the fact that $\zeta (2)= \frac{\pi^2}{6}$ I want a different method. –  Ishan Banerjee Jan 28 '13 at 10:45
    
Ok. I'm not sure of any other way. Hopefully somebody has some insight for you, sorry. –  Rustyn Jan 28 '13 at 10:48

2 Answers 2

up vote 5 down vote accepted

Step I
Integrating by part we get that

$$\int^1_0 \frac{\log(1+x)}{x}dx=-\int^1_0 \frac{\log(x)}{x+1}dx$$

Step II
Letting $x=e^{-u}$, we have $$\int_0^{\infty}\frac{u}{e^u+1}du$$
Step III $$\int_0^{\infty}\frac{u^{s-1}}{e^u+1}du=\Gamma(s)\cdot\eta(s)\tag1$$ that is the product between gamma function and Dirichlet eta function

Step IV
Let $s=2$ in $(1)$ and we're done.

Chris.

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I did go up till step 2 in my attempts. Unfortunately, I've never even heard of the Dirichlet eta function. Is there any other way(using only elementary functions)? –  Ishan Banerjee Jan 28 '13 at 13:39
    
@IshanBanerjee: you may express $1/(e^u+1)$ as a geometrical progression sum that is easy to do. –  Chris's sis Jan 28 '13 at 13:41
    
@ Chris's sister Yes, but then you again need to know the value of $\eta(2)$ from it's series, which is what I wanted to avoid. –  Ishan Banerjee Jan 28 '13 at 13:46
    
@IshanBanerjee: I'm not sure I understand what is your desired way. I also want a proof for Basel problem by only using the knowledge in the middle school. :-) –  Chris's sis Jan 28 '13 at 13:49
    
True. I'll accept it. –  Ishan Banerjee Jan 28 '13 at 13:54

You might be interested in this: noticing that

$$\int_0^1 \frac{1}{1+xy}dy=\frac{\ln (x+1)}{x}$$

We can rewrite the integral as:

$$\int_0^1\frac{\ln (x+1)}{x}\;dx=\int_0^1\int_0^1 \frac{1}{1+xy}\;dy\;dx$$

Now read page 11 of this article (you'll have to slightly adapt the above of course).

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+1. I had forgotten about this useful method. –  Pedro Tamaroff Jan 28 '13 at 18:00

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