Fourier Transform of Differential Equation

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Homework Help Overview

The discussion revolves around a differential equation involving third-order spatial derivatives and their Fourier transforms. The equation is given as \(\frac{\partial^{3}u}{\partial x^{3}} + 2 \left( \frac{\partial u}{\partial x} \right) = \frac{\partial u}{\partial t}\). Participants are tasked with showing that the Fourier transform of the solution can be expressed in a specific form involving an unknown function of \(k\).

Discussion Character

  • Exploratory, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss the process of applying the Fourier transform to the differential equation, questioning how to handle the derivatives correctly. There are attempts to clarify the relationship between spatial derivatives and their Fourier transform representations. Some participants express confusion about the transformations and the implications of the third derivative.

Discussion Status

The discussion is ongoing, with participants providing guidance on the application of Fourier transform properties. There is a recognition of the need to substitute the proposed solution into the original equation to verify its validity. Multiple interpretations of the steps involved are being explored, and some participants are beginning to connect the transformations to the proposed solution form.

Contextual Notes

Participants note the distinction between the original function \(u\) in the differential equation and its Fourier transform \(u(k,t)\). There is also mention of specific initial conditions that may influence the function \(A(k)\), which remains undefined in the current discussion.

  • #31
Right. That's the equation u(k,t) satisfies since, as the others have noted, u(k,t)=F[u(x,t)].
 
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  • #32
(ik)^{3} + 2ik = \frac{\partial}{\partial t}
 
  • #33
Hart said:
(ik)^{3} + 2ik = \frac{\partial}{\partial t}

Unfortunately, \partial/\partial t alone doesn't have any meaning; it's an operator that acts on u(k,t), which can't just be divided away.

You are looking for a function whose time derivative equals the function itself multiplied by -i(k^3-2k).
 
  • #34
well..

e^{-i(k^3-2k)t}

would mean:

\left(\frac{\partial}{\partial t}\right)e^{-i(k^3-2k)t} = -i(k^3-2k)e^{-i(k^3-2k)t}

?!?
 
  • #35
This is what you were hoping to demonstrate at the beginning of this thread, yes?
 
  • #36
.. indeed it is! :biggrin:
 
  • #37
.. any pointers for finding A(k) for the case where u(x,t) at t=0 is given by:

u(x,0) = U_{0}\delta (x-a) where Uo is a constant ?!?
 

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