How to calculate the fourier transform of a gaussion?

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Discussion Overview

The discussion revolves around calculating the Fourier transform of a Gaussian function, specifically the integral \(\int_0^{\infty}e^{-a t^2}\cos(2xt)dt\) and its relation to the Fourier transform of a Gaussian function expressed as \(\int_{-\infty}^{\infty}e^{-a t^2}e^{-i2\pi x t}dt\). Participants seek step-by-step methods for solving this integral.

Discussion Character

  • Exploratory, Technical explanation, Mathematical reasoning

Main Points Raised

  • One participant, Jo, presents the integral and expresses a desire for a step-by-step solution.
  • Another participant suggests using the identity \(\cos(x) = \frac{1}{2}(e^{\mathrm i x} + e^{-\mathrm i x})\) and recommends completing the square and performing a contour integral.
  • Jo acknowledges the problem can be framed as the Fourier transform of a Gaussian and mentions attempting a complex contour integral but feels stuck, asking for more specific guidance.
  • Two participants suggest using the completion of the square and performing direct integration, providing a specific transformation of the integral.
  • Jo recognizes a mistake in their approach after receiving feedback and thanks the participants for their help.

Areas of Agreement / Disagreement

Participants appear to agree on the methods of completing the square and using contour integration, but there is no consensus on the specific steps or techniques to be employed, as Jo expresses uncertainty about the process.

Contextual Notes

Jo's initial approach and understanding of the integral may be limited, and there are unresolved steps in the proposed methods for calculating the integral.

jollage
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Hi all,

I want to calculate [itex]\int_0^{\infty}e^{-a t^2}\cos(2xt)dt=\frac{1}{2}\sqrt{\frac{\pi}{a}}e^{\frac{-x^2}{a}}[/itex]. The answer is known from the literature, but I don't know how to do it step by step. Any one has a clue? Thanks.

Jo
 
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Use [itex]\cos(x) = \frac{1}{2}(e^{\mathrm i x} + e^{-\mathrm i x})[/itex], then complete the squares and perform a countour integral.
 
Hi rubi,

Thank you. Yes, the problem is actually starting in the form of [itex]\int_{-\infty}^{\infty}e^{-a t^2}e^{-i2\pi x t}dt=?[/itex], which is the Fourier transform of a gaussian. I actually tried using complex contour integral, but I was stuck there. Could you be more specific? I guess I don't some tricks...

Thank you

Jo
 
Use the completion of the square and do a direct integration.
at2+i2πxt =a(t +iπx/a)2+(πx)2/a. Calculate the t integral.
 
mathman said:
Use the completion of the square and do a direct integration.
at2+i2πxt =a(t +iπx/a)2+(πx)2/a. Calculate the t integral.

O, I see. Yes, I was not careful. Thank you both.

Jo
 

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