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Possibility to simplify $\sum\limits_{k = - \infty }^\infty {\frac{{{{\left( { - 1} \right)}^k}}}{{a + k}} = \frac{\pi }{{\sin \pi a}}} $

Given that $0 < a < 1$ how to evaluate by the method of residues $ \int_{-\infty}^\infty {e^{ax} \over 1 +e^x } \; dx $

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    I explain it in detail in the question.2012-09-06

2 Answers 2

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Substituting $u=e^x$, so that $dx = \dfrac{du}{u}$, the integral becomes

$\int_0^{\infty} \dfrac{u^{a-1}}{1+u}du$

How might you solve this? You've tagged the question as homework, so I'll leave it here for now, but if you're still stuck, post in the comments.

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    yeah the answer is $ \pi \over \sin (a \pi )$2012-09-05
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Recalling the $\beta$ function,

$ \beta( n,m ) = \int_{0}^{1} u^{n-1}\,(1-u)^{m-1}\, du = \frac{\Gamma(n)\Gamma(m)}{\Gamma(m+n)} \,.$

Using the substitution $ 1+u=\frac{1}{t} $ casts $ \int_0^{\infty} \dfrac{u^{a-1}}{1+u}du $ in terms of the $\beta$ function,

$\int_0^{\infty} \frac{u^{a-1}}{1+u}du = \int _{0}^{1}\!{t}^{-a} \left( 1-t \right) ^{a-1}{dt} =\Gamma(a)\Gamma(1-a) = \frac{\pi}{\sin (a \pi )} \,. $

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    @Downvoter:I do not know what you are downvoting for. I gave a way to evaluate the integral in a way different from the complex techniques.2012-09-05