Evaluate Int: Gaussian Wavepacket & Fourier Transform

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Discussion Overview

The discussion revolves around evaluating a specific integral related to the Fourier transform of a Gaussian wavepacket. The integral involves exponential and cosine functions, with participants exploring methods to simplify and compute it, including contour integration techniques.

Discussion Character

  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant seeks assistance in evaluating the integral of a Gaussian wavepacket, specifically involving exponential and cosine terms.
  • Another participant suggests resolving the cosine function into exponentials and completing the square as a method to approach the integral.
  • A follow-up question seeks clarification on the suggested method of contour shifting and treating the imaginary unit as a parameter.
  • A further response elaborates on the contour integration approach, proposing that the integral can be transformed into a standard Gaussian integral by shifting the contour and arguing that there are no poles present.

Areas of Agreement / Disagreement

Participants express different levels of understanding regarding the suggested methods, with some seeking clarification while others provide technical insights. No consensus is reached on the best approach to evaluate the integral.

Contextual Notes

The discussion includes assumptions about the behavior of integrals involving complex parameters and the applicability of contour integration techniques, which may depend on the specific values of the constants involved.

thegaussian
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I'm doing a Fourier transform of a gaussian wavepacket, so I can get the momentum representation of the wave... To progress I need to evaluate the following integral:

Int{exp[-(sigma^2.x^2 + bx)/4k].cos[(tx^2 - cx)/8k]}dx

with sigma, b,k,t and c all being constants, and the limits being ±infinity.
Any help would be much appreciated!
 
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resolve cos into exponentials and complete the square. Do a contour shift (or just pretend i is just a parameter)
 
tim_lou said:
resolve cos into exponentials and complete the square. Do a contour shift (or just pretend i is just a parameter)

what does that mean?
 
ice109 said:
what does that mean?

well, I believe that if you work out the integral, you'll get something like
\int_{-\infty}^{\infty} e^{-(a+ib)(x-(c+id))^2} dx =\int_C e^{-(a+ib)z^2} dz

where the contour for z is not the real line but shifted by some c+id. One may then argue that since there are no poles anywhere, we can change the contour back to the real line and get a standard gaussian integral. Of course, usually people (at least for me) just pretend i is a real parameter and crank the integral through.
 

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