How Does Changing Variables Simplify Nonlinear PDEs?

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Discussion Overview

The discussion centers around the simplification of a nonlinear partial differential equation (PDE) through the introduction of new variables. Participants explore the transformation of the equation and the implications of using the chain rule in this context.

Discussion Character

  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant presents a nonlinear PDE and seeks clarification on how to derive a new equation by changing variables from (s, t) to (x, t).
  • Another participant suggests using the chain rule to express the derivatives in terms of the new variable x, indicating that both partial derivatives with respect to s and t are equal to 1 when substituting x = s - t.
  • A later reply challenges the effectiveness of the substitution and indicates that it does not yield the desired equation.
  • Another participant claims to have solved the problem using a similar concept of the chain rule but with a different approach, though details of this approach are not provided.

Areas of Agreement / Disagreement

The discussion reflects a lack of consensus, with participants presenting different methods and results regarding the transformation of the PDE. Some approaches are challenged, and alternative methods are suggested without resolution.

Contextual Notes

Participants express uncertainty regarding the effectiveness of their methods and the assumptions underlying their transformations. Specific mathematical steps and dependencies on definitions are noted but remain unresolved.

aquarian11
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I need guidance regarding PDE.
If u have a nonlinear PDE as
Ut+Us+a*U*Us*b*Usss=0
where U is function of (s,t) and a,b are constants.
by introducing new variable x=s-t we will get
Ut+a*U*Ux+b*Uxxx=0
Ut means partial derivative w.r.t time
Us means partial derivative w.r.t s.

How can we get the second equation from the first one?
 
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By using the chain rule.

[tex]U_s= U_t \frac{\partial t}{\partial s}+ U_x\frac{\partial x}{\partial s}[/tex]
Note: if you are going to use x= s- t to replace s only, you will need to think of s as a function of the other variable, t.
If x= s- t, then s= s+ t so both partial derivatives are 1:
[tex]U_s= U_t+ U_x[/itex]<br /> [tex]U_ss= (U_t+ U_x)_s= (U_t+ U_x)_t + (U_t+ U_x)_x= U_tt+ 2U_tx+ U<br /> _xx[/tex]<br /> <br /> Similarly, <br /> [tex]U_sss= U_ttt+ 3Uttx+ 3Utxx+ Uxxx[/tex]<br /> Sustitute those into you equation.[/tex]
 
Subsituting these , will not give me the desired equation.
 
I have solve it, same concept of chain rule but with different approach.
 

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