Integral involving sine and root

Click For Summary
SUMMARY

The integral \(\int_0^{\infty} \frac{\sin(ax)}{\sqrt{x}} dx\) can be solved using a substitution \(u = x^{1/2}\), transforming the integral into one involving \(\sin(a u^2)\). By extending the integral to the entire real axis, the solution can be derived from the imaginary part of \(e^{i a u^2}\). Utilizing the Gaussian integral result, \(\int e^{-a y^2} dy = \sqrt{\frac{\pi}{a}}\) for \(\text{Re}(a) > 0\), provides the final answer, although care must be taken with limits or Wick rotation for \(\text{Re}(a) = 0\).

PREREQUISITES
  • Understanding of improper integrals and convergence
  • Familiarity with integration techniques, particularly substitution
  • Knowledge of complex analysis, specifically the imaginary part of exponential functions
  • Experience with Gaussian integrals and their properties
NEXT STEPS
  • Study advanced integration techniques, focusing on substitution methods
  • Learn about Gaussian integrals and their applications in complex analysis
  • Explore Wick rotation and its role in evaluating integrals
  • Investigate the properties of the imaginary part of complex exponentials
USEFUL FOR

Mathematics students, particularly those studying calculus and complex analysis, as well as researchers dealing with integrals in physics and engineering applications.

zandria
Messages
15
Reaction score
0

Homework Statement



\int_0^{\infty} sin(ax) / sqrt(x) dx

Homework Equations





The Attempt at a Solution


I thought of using integration by parts, but that gets me nowhere. I'm not sure how to go about this problem.
 
Physics news on Phys.org
I have found a way of solving it but I don't know if you will be able to follow, because I don't know what integration techniques you have learned. It's perhaps a bit over-complicated, as well. But I did the integration in Mathematica, and what I got reminded me very much of a Gaussian integral.

If you first do a substitution to u = x1/2 you get an integral over something like sin(a u2). By symmetry you can extend this integral to the entire real axis. Then note that what you want is precisely the imaginary part of ei a u2. Then you can use the result that the integral of exp(-a y2) over the real line is sqrt(pi / a) as long as Re(a) > 0 (technically, you will get Re(a) = 0 so you might need to take some limit, or perform a Wick rotation to make things rigorous) and you will get your answer.

Anyone for an easier approach?
 
Thanks. That's exactly what I needed.
 

Similar threads

  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 8 ·
Replies
8
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 105 ·
4
Replies
105
Views
7K
Replies
8
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 1 ·
Replies
1
Views
955
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 3 ·
Replies
3
Views
1K