Solving ODEs w/ Frobenius Method: Q on b, c as Funcs of x

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    Frobenius Method
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Discussion Overview

The discussion revolves around the application of the Frobenius method for solving ordinary differential equations (ODEs), specifically addressing the conditions under which the method can be applied when coefficients are functions of the independent variable versus when they are constants. Participants explore the implications of these conditions on the solution forms and express challenges in applying the method to specific examples.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant questions whether the Frobenius method can be used when the coefficients b and c are functions of x, contrasting this with the Euler-Cauchy equation where b and c are constants.
  • Another participant asserts that the form y = x^r is only applicable for the Euler-Cauchy equation, indicating that if b and c are functions of x, this form cannot be used.
  • A participant expresses difficulty in applying the Frobenius method to a specific ODE, citing a lack of clarity in their textbook regarding the next steps after deriving a series solution.
  • Discussion includes the identification of x = 0 as a regular singular point and mentions the indicial equation r^2 + 4r = 0, yielding roots r1 = 0 and r2 = -4, with a note on the implications of these roots for the Frobenius method.
  • One participant suggests a substitution related to Bessel equations as a potential approach to solving the ODE in question.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of the Frobenius method based on the nature of the coefficients b and c. There is no consensus on the best approach to take for the specific ODE presented, and challenges in applying the method remain unresolved.

Contextual Notes

Participants note limitations in their textbook's explanations, which may affect their understanding of the Frobenius method's application. The discussion also highlights the complexity of the indicial roots and their implications for finding solutions.

Master J
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I have been looking at the Frobenius method for solving ODEs of the form. I have a few questions on it.


(x^2)y'' + xby' + cy = 0

If b and c are functions of x, does one use the Frobenius method, where as if they are constants, it is an Euler Cauchy equation and you use y = x^r ??

Thats the first Q. anyway.

Thanks folks!:smile:
 
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y=xr only works for the Euler-Cauchy equation, i.e. b and c are constants.

If b and c are functions of x, you cannot use the try function y=xr.
 
Thanks for cledaring that up.

I'm having trouble applying the method. My textbook, (which I won't name but it's approach and exlanation in this section is absolutely terrible) isn't helping me much.

I have been trying to solve, for example,

xy'' + 5y' + xy = 0

So I get

SUM(n + r)(N + r -1)(a_n)x^(n + r -2) + SUM(5)(n + r)(a_n)x^(n + R -2) + SUM(a_n)x^(n + r +1) = 0

where SUM is the sum to infinity from n = 0.

and the general solution is of form y=(x^r)SUM(a_n)(x^n)


I don't know what to do know. The book's next steps are done without explanation really.
Can someone help me??
 
x=0 is a regular singular point. There is at least one solution of the form y=(x^r)SUM(a_n)(x^n)
where r satisfy the indicial equation r2+4r=0.
r1=0 , r2=-4 and r1-r2 is an integer in this case.

The Frobenius method only guarantee for r=0 (the larger root) but not for r=-4 (but there is no harm for trying)

If you know anything about Bessel equation, I would suggest you solve the equation using the substitution z=x2y.
 

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