Evaluating $\int_{0}^{\infty} (\frac{\sin x}{x})^2 dx$ using complex analysis

calculuscomplex-analysisimproper-integrals

I need to calculate $\displaystyle\int_{0}^{\infty} \left(\frac{\sin x}{x}\right)^2dx$.

I have started with defining:
$$f(z) = \frac{1-e^{2iz}}{z^2},\quad z\in\mathbb{C}\;.
$$

Then divided it into four contour integrals, just standard stuff:
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$\int_{-R}^{-r} \frac{1-e^{2ix}}{x^2} dx + \int_{r}^{R} \frac{1-e^{2ix}}{x^2} dx + \int_{C_r }^{} \frac{1-e^{2iz}}{z^2} dz+\int_{C_R }^{} \frac{1-e^{2iz}}{z^2} dz$

First two integrals add up to $2\int_{r}^{R} \frac{1-\cos(2x)}{x^2} dx$

Third integral, Laurent series for $\frac{1-e^{2iz}}{z^2} = \frac{-2i}{z} +2 + \frac{4iz}{3} – \frac{2z^2}{3} +… = \frac{-2i}{z} + P(z)$ and $\int_{C_r} P(z) = 0$ as $r \to 0$, so:

$\int \frac{-2i}{z}dz=-\int_0^\pi \frac{2i^2re^({ti})}{re^{ti}}dt = 2\pi$ for $z = re^{ti}$

$\int_{C_R }^{} \frac{1-e^{2iz}}{z^2} dz \leq \frac{2}{R} +\frac{|e^{2xi}|e^{-2y}}{|z^2|} \leq \frac{2+\pi}{R}= 0$ as $R \to \infty$

$2\int_{0}^{\infty} \frac{1-\cos(2x)}{x^2} dx + 2\pi = 4\pi$ so my integral is equal to $\pi$, whereas it should be $\frac{\pi}{2}$. Where did I go wrong?

Best Answer

$$1-\cos{2x}=2\sin^2{x}$$ You're missing the $\frac{1}{2}$ when you converted to exponential form.

I believe another mistake you made is that the bounds for the line integral of the semi-circle with little radius r should be $\pi$ to $0$ because it should be in the clockwise direction. You would get $-2\pi$ from that instead of $2 \pi$. Then, you should use Cauchy's first theorem instead of second because the semi-circle of little radius r avoids the singularity: $$ 2\int_0^{\infty} \frac{1-\cos{2x}}{x^2} -2\pi=0$$ $$\int_0^{\infty} \frac{\sin^2{x}}{x^2} \; dx= \boxed{\frac{\pi}{2}}$$

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