Why is the method of characteristics used to solve this PDE?

Click For Summary

Discussion Overview

The discussion revolves around the method of characteristics as applied to a specific partial differential equation (PDE): xUx + yUy = nu. Participants explore the solution form u(x,y) = x^n f(y/x) and question the derivation and correctness of this solution, as well as the rationale behind using the method of characteristics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents the equation and its proposed solution, expressing confusion about the role of the term x^n in the solution.
  • Another participant suggests their own solution yielded a different term, y^n, and questions the addition versus multiplication of terms in the solution.
  • A third participant emphasizes the importance of verifying solutions by substituting them back into the original equation, suggesting that the mathematician who derived the solution may have had specific insights into the form of the solution.
  • A later reply critiques the method used to find characteristics, stating it is not generally correct and attributing the correct characteristic equation to chance.

Areas of Agreement / Disagreement

Participants express differing views on the derivation of the solution and the validity of the methods employed. There is no consensus on the correctness of the proposed solutions or the method of characteristics.

Contextual Notes

Some participants note limitations in the methods used to derive characteristics and the potential for error in the solutions presented. The discussion reflects uncertainty regarding the correctness of the approaches taken.

Who May Find This Useful

Readers interested in partial differential equations, the method of characteristics, and solution verification may find this discussion relevant.

chaotixmonjuish
Messages
284
Reaction score
0
I borrowed this book form the library to get a heads up on what i would be doing in PDEs and it had this equation

xUx+yUy=nu

and its solution (according to the back) is: u(x,y)=xnf(y/x)

I did figure out the f(y/x) part:

dx/x=dy/y
ln(x)+c=ln(y)
cx+y
c=y/x


how did the xn come into play
 
Physics news on Phys.org
My attempt at a solution yielded a yn. I don't believe that is wrong, but I am wondering why they are multiplied together instead of added.
 
You ofcourse can check your or the book's answer by plugging it back to the equation.
I think that there is no easy way here, the mathematician who find these equations knew already what solution he was seeking and just played with its derivatives to get to this equation.
I don't have time to check your work, but have you plugged your answers back to the eqaution to check that it's valid?
 
chaotixmonjuish said:
I borrowed this book form the library to get a heads up on what i would be doing in PDEs and it had this equation

xUx+yUy=nu

and its solution (according to the back) is: u(x,y)=xnf(y/x)

I did figure out the f(y/x) part:

dx/x=dy/y
ln(x)+c=ln(y)
cx+y
c=y/x


how did the xn come into play
The form of the question suggests that you should use the method of characteristics (see http://www.stanford.edu/class/math220a/handouts/firstorder.pdf" for more information). Note that the method you have used above to find the characteristics is not generally correct and it is just by 'chance' that you have arrived at the correct characteristic equation (c=y/x).

The link I provided above should give you everything you need, if not, I suggest reading the preceeding chapter of your book.
 
Last edited by a moderator:

Similar threads

  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 2 ·
Replies
2
Views
4K
  • · Replies 5 ·
Replies
5
Views
3K
  • · Replies 3 ·
Replies
3
Views
4K
  • · Replies 16 ·
Replies
16
Views
4K
  • · Replies 36 ·
2
Replies
36
Views
6K
  • · Replies 18 ·
Replies
18
Views
3K
  • · Replies 0 ·
Replies
0
Views
4K
  • · Replies 5 ·
Replies
5
Views
2K