Trigonometric integral / Complex Analysis

In summary, the conversation is about calculating the integral \int\limits_0^{\infty} \cos{x^2} dx using complex analysis and the Gaussian integral. The participant is unsure of how to approach the problem, but another participant suggests using the Gaussian integral and provides a formula for it. The conversation also mentions that the Gaussian integral holds for all values of a where Re(a) > 0, and may also hold for certain values when Re(a) = 0 and Im(a) is non-zero.
  • #1
irycio
97
1

Homework Statement



Calculate the integral [tex] \int\limits_0^{\infty} \cos{x^2} dx [/tex]

This is the exercise from complex analysis chapter, so I guess I should change it into a complex integral somehow and than integrate. I just don't know how, since neither substitution cos(x^2) = Re{e^ix^2} nor cos(x^2)=1/2(e^ix^2 + e^-(ix^2)) works.

Thanks in advance for your help :)
 
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  • #2
Do you know the Gaussian integral
[tex]\int_{-\infty}^\infty e^{-a x^2} \, dx = \sqrt{\frac{\pi}{a}}[/tex]

It formally applies to all a for which Re(a) > 0, although as far as I am aware it continues to hold when Re(a) goes to 0, if Im(a) is non-zero, i.e.
[tex]\lim_{\epsilon \downarrow 0} \int_{-\infty}^\infty e^{-(\epsilon + i a) x^2} \, dx = \sqrt{\frac{\pi}{ ia}} = (1 - i) \sqrt{\frac{\pi}{a}} [/tex]
 

1. What is a trigonometric integral?

A trigonometric integral is an integral that involves trigonometric functions, such as sine, cosine, tangent, etc. These integrals can be solved using various techniques, including substitution, integration by parts, and trigonometric identities.

2. How is complex analysis related to trigonometric integrals?

Complex analysis is a branch of mathematics that deals with the properties and behavior of complex numbers. Trigonometric integrals can often be solved using complex analysis techniques, such as contour integration, Cauchy's integral theorem, and residue calculus.

3. Can trigonometric integrals be solved using basic integration rules?

Some simple trigonometric integrals can be solved using basic integration rules, such as the power rule or the substitution method. However, more complex integrals may require the use of specialized techniques, such as those from complex analysis.

4. What are some applications of trigonometric integrals?

Trigonometric integrals have many applications in various fields, including physics, engineering, and statistics. They are commonly used to solve problems involving periodic functions, such as in calculating the area under a curve or finding the average value of a function.

5. Are there any common mistakes to watch out for when solving trigonometric integrals?

One common mistake when solving trigonometric integrals is forgetting to use the correct trigonometric identities or substitution techniques. It is also important to be mindful of the domain of the integral and any potential discontinuities in the integrand. It is always helpful to double-check your work and make sure the final result makes sense in the context of the problem.

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