Sin(x)/x via contour integration

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In summary, to evaluate the integral \int_{0}^{\infty} \frac{\sin x}{x}\,dx, we can use Cauchy's integral theorem and the contour shown in the hint to rewrite it as a sum of integrals over semicircles. By applying Euler's formula and using Lebesgue's dominated convergence theorem, we can show that these integrals approach 0 as the radius of the semicircles approaches infinity and the distance between them approaches 0. Therefore, the only contribution to the integral comes from the real axis, giving the result \int_{-\infty}^{\infty} \frac{e^{ix}-1}{x} dx = \pi i.
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snipez90
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Homework Statement


Evaluate [itex]\int_{0}^{\infty} \frac{\sin x}{x}\,dx[/itex]

Homework Equations



1. Cauchy's integral theorem
2. [tex]\int_{|z| = r} \frac{dz}{z} = 2\pi i[/tex]

The Attempt at a Solution


I want to see if the following works. The hint suggests the use of the contour here: http://upload.wikimedia.org/wikibooks/en/9/9e/ContourSinzz.gif" . We denote by [itex]\gamma _R ^+[/itex] and [itex]\gamma _{\epsilon} ^+[/itex] the semicircles of radius R and [itex]\epsilon[/itex] with positive and negative orientations, respectively.

We consider the function
[tex]f(z) = \frac{e^{iz}-1}{z}.[/tex]
By Cauchy's theorem, we have
[tex]\int_{-R}^{-\epsilon} + \int_{\gamma _{\epsilon} ^+} + \int_{\epsilon}^{R} + \int_{\gamma _{R} ^+}.[/tex]

First we note that
[tex]\int_{\gamma _{\epsilon} ^+} f(z)\, dz = \int_{\gamma _{\epsilon} ^+} \frac{e^{iz}}{z}\, dz \,-\, \int_{\gamma _{\epsilon} ^+} \frac{dz}{z} = \int_{\pi}^{0} \frac{\exp \left(i \epsilon e^{it} \right)}{\epsilon e^{it}}\cdot i \epsilon e^{it} dt \,+\, \pi i = \int_{\pi}^{0} i\exp \left(i\epsilon e^{it}\right)dt + \pi i.[/tex]

Since [itex]|e^{iz}| = e^{-y},[/itex] applying Euler's formula gives
[tex] |i\exp{(i\epsilon e^{it})}| \leq e^{-\epsilon \sin t}.[/tex]
Letting [itex]\epsilon \rightarrow 0[/itex] and applying Lebesgue's dominated convergence theorem, we have
[tex]\lim_{\epsilon \rightarrow 0}\int_{\gamma _{\epsilon} ^+} f(z)\, dz = \lim_{\epsilon \rightarrow 0} \int_{\pi}^{0} i\exp \left(i\epsilon e^{it}\right) dt + \pi i = 0.[/tex]

Applying the same arguments to the larger semicircle, we have
[tex]\left|\int_{\gamma _{\R} ^+} \frac{e^{iz}}{z}\,dz\right| = \left|\int_{0}^{\pi} i\exp \left(iRe^{it} \right) dt \right| \leq \int_{0}^{\pi} \exp(-R \sin t)\,dt.[/tex]
Clearly this last integrand is dominated over the interval [itex][0,\pi][/itex], so applying DCT again shows that this last integral vanishes as [itex]R \rightarrow \infty.[/itex]

Thus the contribution from the integrals over each semicircle is [itex]-\pi i[/itex] as we let R approach infinity and epsilon approach 0. Hence
[tex]\int_{-\infty}^{\infty} \frac{e^{ix}-1}{x} dx = \pi i.[/tex]
Taking imaginary parts and noting parity symmetry gives the desired result.

*EDIT* Hopefully I'm not missing anymore differentials.
 
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The solution is correct. Another way to show ##\lim\limits_{\epsilon \to 0} \int_{\gamma_{\epsilon}^+} f(z)\, dz = 0## is to note that since ##f(z)## has a removable singularity at the origin, it is bounded near the origin and hence ##\int_{\gamma_{\epsilon}^+} f(z)## is bounded by a constant times the arclength of ##\gamma_\epsilon^+##, which is a constant times ##\epsilon##. Therefore ##\lim\limits_{\epsilon \to 0} \int_{\gamma_{\epsilon}^+} f(z)\, dz = 0##, as desired.
 
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FAQ: Sin(x)/x via contour integration

1. What is contour integration?

Contour integration is a mathematical technique used to evaluate integrals along a path in the complex plane. This method allows us to solve complex integrals that cannot be solved using traditional integration methods.

2. How is contour integration used to evaluate Sin(x)/x?

In the case of Sin(x)/x, contour integration is used to evaluate the integral along a semicircle in the complex plane. This allows us to avoid the singularity at x=0 and solve the integral using the residue theorem.

3. What is the significance of Sin(x)/x in mathematics?

Sin(x)/x is a special function known as the sinc function, which has applications in signal processing, Fourier analysis, and other areas of mathematics. It is also a fundamental function in the study of Fourier transforms and their properties.

4. Can contour integration be used to evaluate other integrals?

Yes, contour integration can be used to evaluate a wide range of complex integrals. It is a powerful tool in complex analysis and has applications in various areas of mathematics and physics.

5. Are there any limitations to using contour integration to evaluate integrals?

Contour integration can only be used to evaluate integrals that can be expressed as a sum of residues. It may not be applicable to integrals with infinitely many singularities or those that cannot be represented as a closed contour in the complex plane.

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