Kortweg-de Vries: Parabolic PDE Homework

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Homework Help Overview

The discussion revolves around the classification of the equation ut + uux + uxxx = 0 as a parabolic partial differential equation (PDE). Participants are exploring the necessary transformations and conditions to demonstrate this classification.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to convert the given PDE into a system of first-order equations by introducing an auxiliary variable p = ux. They express uncertainty about their approach and whether they have made an error. Other participants suggest introducing an additional variable v to form a system, prompting questions about the utility of this approach and requests for further clarification.

Discussion Status

The discussion is active, with participants providing guidance on how to structure the problem into a system of equations. There is an ongoing exploration of the implications of these transformations for classifying the PDE, but no consensus has been reached on the best approach yet.

Contextual Notes

Participants are working under the constraints of homework guidelines, which may limit the types of solutions or methods they can employ. The original poster expresses concern about potentially misunderstanding the requirements of the problem.

iamkratos
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Homework Statement



The equation is ut + uux + uxxx = 0

I need to show that this is a parabolic pde.

Homework Equations



Hint : convert to an equivalent system of 1st order equations by introducing an auxiliary variable p = ux, etc.

The Attempt at a Solution



So i took p = ux

doesn't that just give me:

ut + pu + pxx = 0

This i think is a parabolic pde by inspection.

But the hint says i need to get a system of 1st order equations. What am i missing? I am pretty sure I've made a giant error. Help please!
 
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So introduce another variable v=\partial_{x}p and you have a system.
 
How would that help? I don't get it.
Can you please elaborate?
 
Your system of equations is:
<br /> \begin{array}{ccc}<br /> v &amp; = &amp; \partial_{p} \\<br /> p &amp; = &amp; \partial_{x}u \\<br /> \partial_{t}u+pu+\partial_{x}v &amp; = &amp; 0<br /> \end{array}<br />

This system can be written in the form:

<br /> \mathbf{A}\partial_{t}\mathbf{U}+\mathbf{B}\partial_{x}\mathbf{U}=\mathbf{c}<br />

Now the condition for parabolic equation comes in with the determinants of A and B (look this up, this should be in your notes)
 

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