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I am trying to evaluate the following definite integral:

$\int_{-\infty}^\infty \frac{ab\operatorname{sinc}^2(cx)}{a+b\operatorname{sinc}^2(cx)}\;dx$

where $\operatorname{sinc}(x)=\dfrac{\sin x}{x}$ is the Sinc function and $a$, $b$, and $c$ are positive constants. I would also be happy with a reasonably tight upper bound. Does anyone have any ideas?

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    @Bullmoose: The lecture notes of William Chen are$a$good start: http://rutherglen.science.mq.edu.au/wchen/lnicafolder/lnica.html. What you really need is just chapter 11, but, probably, you'll need the other chapters for background.2012-03-02

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I an assuming $a,b,c>0$. Let $\delta=\sqrt{b/a}$. Then $ \int_{-\infty}^\infty \frac{a\,b\,\operatorname{sinc}^2(c\,x)}{a+b\,\operatorname{sinc}^2(c\,x)}\,dx=\frac{b}{c}\int_{-\infty}^\infty\frac{\sin^2x}{x^2+\delta^2\sin^2x}\,dx=\frac{b}{c}\,I(\delta). $ Integrating the inequalities $ \frac{\sin^2x}{x^2+\delta^2}\le\frac{\sin^2x}{x^2+\delta^2\sin^2x}\le\frac{\sin^2x}{x^2} $ we get $ \frac{1-e^{-2\delta}}{2\,\delta}\,\pi\le I(\delta)\le\pi. $ This bounds are good only for for small $\delta$. For Large $\delta$ try the following: $ \begin{align*} I(\delta)&=2\int_0^\delta\frac{\sin^2x}{x^2+\delta^2\sin^2x}\,dx+2\int_\delta^\infty\frac{\sin^2x}{x^2+\delta^2\sin^2x}\,dx\\ &\le\frac{2\,\delta}{1+\delta^2}+2\int_\delta^\infty\frac{\sin^2x}{x^2}\,dx\\ &=\frac{2\,\delta }{\delta ^2+1}+\frac{1-\cos2\,\delta}{\delta}+{\pi -2\,\operatorname{Si}(2\,\delta )}, \end{align*} $ where $\operatorname{Si}$ is the sine integral. in the graph, $I(\delta)$ is in red and the upper and lower bound in blue.

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    $x^2\ge\sin^2x$.2012-03-02