Another question dealing with Frobenius method

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    Frobenius Method
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Discussion Overview

The discussion revolves around a problem related to the Frobenius method in differential equations, specifically concerning the analytic properties of solutions and their coefficients in a punctured neighborhood of the origin. Participants explore the implications of a given solution form and the conditions under which certain functions can be deduced as solutions.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents a problem involving a differential equation with analytic coefficients and a proposed solution involving a logarithmic term.
  • Another participant suggests making a substitution into the differential equation to verify the solution.
  • A different participant expresses difficulty with the substitution, noting that it leads to a complex expression with terms that do not seem to vanish.
  • One participant challenges the initial assumption by providing a counterexample where the coefficients are zero, questioning the validity of the deduction.
  • Another participant proposes that the requirement for poles at the origin is crucial for the argument to hold.
  • A later reply emphasizes that being analytic everywhere implies being analytic in a punctured neighborhood, suggesting that the original argument may still be valid.
  • One participant acknowledges the previous point and expresses agreement with the revised understanding.

Areas of Agreement / Disagreement

The discussion contains multiple competing views regarding the conditions under which the function f can be deduced as a solution. While some participants agree on the implications of analyticity, others present counterexamples that challenge the initial assumptions, leaving the discussion unresolved.

Contextual Notes

Participants highlight the importance of the coefficients' behavior at the origin and the nature of the proposed solution, indicating that the discussion may depend on specific definitions and assumptions about analyticity and singularities.

meteorologist1
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This is another question I have trouble proving:

Suppose the coefficients of the equation: w'' + p(z)w' + q(z)w = 0 are analytic and single-valued in a punctured neighborhood of the origin. Suppose it is known that the function w(z) = f(z) ln z is a solution, where f is analytic and single-valued in a punctured neighborhood. Deduce that f is also a solution.

Thanks for your help.
 
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Can't you just make that substitution into the DE?
 
When I make the substitution it becomes very messy and it is not easy to see that f is a solution. There are terms without the natural log which I can't see how they would vanish.
 
Actually, it's clearly not true. Take p(z) = q(z) = 0. Try the solution w(z) = 1.
 
Hmmm, it's probably because p and q are required to have poles at z = 0. p(z) = q(z) = 0 is analytic at z = 0.
 
The nice thing about being everywhere analytic is that you're also analytic on a punctured neighborhood of the origin. :smile: Whether they're analytic at the origin or not is irrelevant.

If you're still not convinced, try making your own differential equation whose coefficients are singular at the origin. Mine had 1/z as a solution.
 
Yes I agree with you now. Thanks.
 

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