December 19th, 2016, 04:25 PM  #1 
Member Joined: Dec 2016 From:  Posts: 54 Thanks: 10  solve this integral?
Hi, I was looking to solve the following integral \begin{eqnarray} I=\int dx \frac{1}{x^{2}+a^{2}}\frac{1}{(x+b)^{2}+a^{2}} \end{eqnarray} which is the product of two different Lorentzians. Of course, a and b are constants. My initial idea is to spit each Lorentzian into a sum of two terms, like: \begin{eqnarray} \frac{1}{x^{2}+a^{2}}=\frac{1}{2ai}\bigg(\frac{1}{ xia}\frac{1}{x+ia}\bigg) \end{eqnarray} and the same with the other one, if that might simplify things a bit but I dunno. Is there any closed formula for this? Cheers! 
December 19th, 2016, 04:47 PM  #2  
Senior Member Joined: Sep 2015 From: USA Posts: 1,652 Thanks: 839  Quote:
I=\int dx \frac{1}{x^{2}+a^{2}}\frac{1}{(x+b)^{2}+a^{2}} \end{eqnarray}= \Large \frac{a \log \left(a^2+b^2+2 b x+x^2\right)a \log \left(a^2+x^2\right)+b \tan ^{1}\left(\frac{x}{a}\right)+b \tan ^{1}\left(\frac{b+x}{a}\right)}{4 a^3 b+a b^3}$  
December 19th, 2016, 05:22 PM  #3 
Math Team Joined: Jan 2015 From: Alabama Posts: 2,875 Thanks: 766 
ietzsche, do you now understand how romsek got that? Looks like a candidate for "partial fractions".
Last edited by Country Boy; December 19th, 2016 at 05:24 PM. 
December 19th, 2016, 05:42 PM  #4  
Senior Member Joined: Sep 2015 From: USA Posts: 1,652 Thanks: 839  Quote:
I just dumped it into Mathematica and it got spit out. He just asked for a formula, not how to arrive at it.  

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