How to determine particular solutions for cauchy euler

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    Cauchy Euler
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Discussion Overview

The discussion revolves around determining particular solutions for non-homogeneous Cauchy-Euler equations. Participants express confusion regarding the approach to finding these solutions and the assignment of particular solution forms, seeking a general explanation that encompasses various cases.

Discussion Character

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

Main Points Raised

  • One participant questions whether the approach to finding a particular solution for a non-homogeneous Cauchy-Euler equation differs from standard methods.
  • Another participant suggests that substituting ##x = e^t## transforms the Cauchy-Euler equation into a constant coefficient differential equation, which can then be solved using familiar methods.
  • A specific example is provided where the substitution is applied to the equation $$x^2y''(x) + xy'(x) + 4y(x) = \cos(2\ln x)$$, leading to a simpler equation in terms of ##t##.
  • A third participant mentions the method of "variation of parameters" as a general technique for finding particular solutions in linear ordinary differential equations, including Cauchy-Euler equations.
  • One participant acknowledges the effectiveness of the substitution method while noting that variation of parameters can be challenging with certain non-homogeneous terms.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and approaches to the problem, indicating that multiple competing views remain regarding the best method for finding particular solutions in Cauchy-Euler equations. The discussion does not reach a consensus on a singular approach.

Contextual Notes

Participants highlight the complexity of assigning particular solution forms and the challenges posed by specific non-homogeneous terms, indicating that there may be limitations in the general applicability of the discussed methods.

ericm1234
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If given a cauchy euler equation (non-homogeneous) equation, does the approach in looking for a particular solution (in order to solve the non-homogeneous part), differ from normal?
I am also in general confused about how to assign a particular solution form, in many cases. I have yet to find a good, general explanation for how this is determined, covering all cases.
 
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ericm1234 said:
If given a cauchy euler equation (non-homogeneous) equation, does the approach in looking for a particular solution (in order to solve the non-homogeneous part), differ from normal?
I am also in general confused about how to assign a particular solution form, in many cases. I have yet to find a good, general explanation for how this is determined, covering all cases.

These equations are closely related to constant coefficient equations. The substitution ##x = e^t## in a Cauchy-Euler equation in x will change it into a constant coefficient DE in ##t##. You can solve that with the usual methods, then convert back with ##t =\ln x##. Given the few times this comes up, it is probably just as well to do it that way instead of memorizing the corresponding rules for the C-E equation. For example, take the equation$$
x^2y''(x) + xy'(x) + 4y(x) = \cos(2\ln x)$$The substitution ##x=e^t## gives this equation in ##t##:$$
y''(t) + 4y(t) = \cos(2t)$$(If you don't know how to do that, I can expand on it). This is one where the NH term contains part of the complementary solution. But the point is, you probably know how to handle it, right? So solve this for ##y(t)## and substitute ##t = \ln x## to get the solution to the original DE.
 
A general technique for finding the particular solution of a linear ode (such as Cauchy-Euler) is the method of "variation of parameters". Any differential equations book should discuss this - google probably will find something as well. Good luck!

jason
 
Ok thanks guys, that substitution idea works pretty well actually.
VOP can sometimes be very difficult with certain non-homogeneous terms in these CE problems.
 

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