Approximate solution of differential equation

Click For Summary

Discussion Overview

The discussion revolves around the conditions under which a function g(x) can be considered an approximate solution to a differential equation F(y'',y',y,x)=0, given that F(g'',g',g,x)=δ, where δ is small. The scope includes theoretical considerations of differential equations and the behavior of solutions.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions whether g(x) can be an approximate solution, suggesting that the behavior of F is crucial to this determination.
  • Another participant notes that if F is "well behaved," then g making F close to 0 should also be close to the actual solution y that makes F equal to 0, but this is not guaranteed for all functions.
  • A further contribution highlights that even without δ, different initial conditions can lead to vastly different solutions, using the example of y''+y=0 having solutions y=sin(x) and y=100*sin(x).
  • A later reply inquires about the specific case where F resembles the Sturm-Liouville form, asking if such a form would be considered "well behaved" and if the approximation would hold at the singularity point (x=0).

Areas of Agreement / Disagreement

Participants express differing views on what constitutes a "well behaved" function and the implications for approximations. The discussion remains unresolved regarding the specific conditions under which g(x) can be considered an approximate solution.

Contextual Notes

Participants mention the importance of continuity and differentiability of functions in the context of the approximation, as well as the role of initial conditions in determining solutions.

zhanhai
Messages
66
Reaction score
0
Differential equation: F(y'',y',y,x)=0,
y=y(x).

Now, there is g=g(x) with F(g'',g',g,x)=δ, where δ is small. Then, can g(x) be taken as an approximate solution of F(y'',y',y,x)=0?
 
Physics news on Phys.org
That depends strongly on F. As long as F is "well behaved", g making F close to 0 will itself be close to y that makes F equal to 0. However, there will be some functions, that are not continuous or not differentiable or not "sufficiently differentiable", such that this is not true. That is, that a function, g, that makes F small may be wildly different from y that makes F 0.
 
y''+y=0 has the solution y=sin(x). It also has the solution y=100*sin(x) which is completely different.
Even without δ, you can get wildly different results. You have to fix initial conditions to get something like that.
 
To https://www.physicsforums.com/members/hallsofivy.331/:

Thank you for your reply!

Suppose F is similar to Sturm-Liouville form:
y''+p(x)y'+q(x)y=h(x),
where p(x) has no more than first-order singularity point, and q(x) and h(x) each actually has no singularity; then, is F "well behaved"? And, in such case, would the approximation be valid for the only singularity point (x=0)?
 
Last edited:

Similar threads

Replies
1
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 3 ·
Replies
3
Views
4K
  • · Replies 1 ·
Replies
1
Views
1K
  • · Replies 5 ·
Replies
5
Views
4K
  • · Replies 2 ·
Replies
2
Views
4K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 0 ·
Replies
0
Views
4K
  • · Replies 2 ·
Replies
2
Views
2K