Monday 12 November 2018

calculus - How to evaluate $int_{0}^{1} frac{ln x}{x+c} dx$



For $c=-1$ , it can be evaluated using the Taylor Series for $\ln x$ centered at $1$ to get $\zeta (2)$. For $c=1$ you enforce the substitution $x=1-u$ then use the Taylor series centered at $0$.





Can we generalize for $c \in \mathbb{R}$?




I've completed calc 1 and 2 but currently taking no math classes besides Ap statistics. I try to use this website to expand my math toolbox and learn tricks here and there. So can someone incorporate a technique that's not to far away for me to understand.



Thanks.



Idea for $c<0$:




Taylor series for $\ln x$ about $x=-c$ is:



$$\ln (x)= \ln (-c)+ \sum_{n=1}^{\infty} \frac{(n-1)!(-1)^{n+1}}{(-c)^n} \frac{(x+c)^n}{n!}$$



I'm good from there... divide by $(x+c)$ and integrate.. and I'm left with a pretty weird sum, which I see as acceptable. Here's what I've got from this method:
$$\ln(-c) \int_{0}^{1} \frac{dx}{x+c}-\sum_{n=1}^{\infty} \frac{(1+c)^n-c^n}{n^2c^n}$$



Which can be simplified if $c \notin (-1,0)$




Now all that is left is $c \geq 0$


Answer



The Dilogarithm function can be defined as $$-\int_{0}^{1}\frac{\log\left(1-xt\right)}{x}dx=\textrm{Li}_{2}\left(t\right)
$$ so in your case we have, taking $t=-\frac{1}{c},c\notin\left(-1,0\right]
$ and integrating by parts, $$\textrm{Li}_{2}\left(-\frac{1}{c}\right)=-\int_{0}^{1}\frac{\log\left(1+x/c\right)}{x}dx$$ $$=-\left[\log\left(x\right)\log\left(1+x/c\right)\right]_{0}^{1}+\int_{0}^{1}\frac{\log\left(x\right)}{x+c}dx=\int_{0}^{1}\frac{\log\left(x\right)}{x+c}dx.$$


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