Integral of x^(m-1)/(1+x) wrt x?

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Discussion Overview

The discussion revolves around the integral \(\int_0^\infty\frac{x^{m-1}}{1+x} dx\) and its purported equality to \(\frac{\pi}{\sin(m\pi)}\). Participants explore methods to prove this relationship, its implications in the context of the Euler reflection formula, and the challenges associated with finding an elementary primitive for the integral.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions the validity of the equality and expresses difficulty in proving it through standard methods such as substitution and integration by parts.
  • Another participant suggests testing special cases for \(m\) (e.g., \(m=1/2\), \(m=3/2\), \(m=1/4\)) and analyzing the domain of convergence of the integral.
  • A participant notes that there may not be an elementary primitive for the integral and references the relationship to the product \(\Gamma(m)\Gamma(1-m)\).
  • Further references to external resources are provided, including a link to a document discussing the Gamma function and the Euler reflection formula.
  • One participant indicates a willingness to accept the result as a fact, acknowledging the difficulty in deriving it without advanced techniques.
  • Another participant suggests that contour integration in the complex domain might be a useful approach to prove the relationship.

Areas of Agreement / Disagreement

Participants express varying levels of confidence in the equality, with some accepting it as a fact while others seek proof and clarification. There is no consensus on a definitive method to establish the equality or on the existence of an elementary primitive.

Contextual Notes

Participants highlight the potential complexity of the integral and the need for advanced mathematical tools, such as the Gamma function and contour integration, to explore the relationship further. The discussion also reflects uncertainty regarding the convergence of the integral for different values of \(m\).

s0ft
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How does this:
\int_0^\infty\frac{x^{m-1}}{1+x} dx
equal
\frac{\pi}{sin(m\pi)}
?
It has been simply stated as a fact in a proof of the so called "Euler reflection formula" in a textbook.
I have tried the usual ways, substitution, integration by parts and even series expansion of 1/1+x but I can't find how the above equality is true.
 
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I suggest you to first try on some special cases.
For example: m=1/2, m=3/2, m=1/4.
Then, also, analyze the domain of convergence of this integral.
Besides, I think there is no elementary primitive.
Have a look on the wiki article: http://en.wikipedia.org/wiki/Reflection_formula .
Maybe you might prove the relation of your integral to the product \Gamma(m)\Gamma(1-m)
 
Far from obvious!
See pages 13-17

http://warwickmaths.org/files/gamma.pdf
 
Last edited by a moderator:
Thanks. So I guess I will just have to accept that as a fact. No easy and 'real' way to obtain the result without resorting to other forms of Gamma function.
 

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