Solving the ODE with Arbitrary Constants: A Search for Analytic Solutions

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Discussion Overview

The discussion revolves around finding an analytic solution to a specific ordinary differential equation (ODE) involving arbitrary constants. Participants explore various methods and substitutions to approach the problem, which remains unresolved regarding the existence of a general solution.

Discussion Character

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

Main Points Raised

  • One participant expresses doubt about the existence of an analytic solution to the ODE: \(\ddot{f} + b \tan(b t) \dot{f} - a^2 \cos^2(b t) f = 0\).
  • Another participant suggests rewriting the tangent function as sine over cosine and multiplying the equation by \(\cos(bt)\), proposing a substitution of the variable to \(\tau = \sin(bt)\).
  • A later reply indicates that a different substitution, \(\tau = \cos(bt)\), is more effective, leading to a solution via a power series method.
  • There is a suggestion that the ODE can be solved exactly using the initial substitution proposed by the second participant.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the existence of an analytic solution, with multiple approaches and substitutions being discussed without agreement on a definitive method or outcome.

Contextual Notes

The discussion includes various assumptions about the substitutions and methods used, but these assumptions are not fully explored or resolved. The effectiveness of the proposed methods may depend on specific conditions or interpretations of the ODE.

MathNerd
I’ve been trying to find an analytic solution to the following ODE. I haven’t been successful and have come to the conclusion that an analytic solution probably doesn’t exist. I am not totally sure though and would be appreciative if you guys gave it a look.

a & b are arbitrary constants...

[tex] \ddot{f} + b tan(b t) \dot{f} - a^2 cos^2(b t) f = 0[/tex]

Thanks in advance...
 
Last edited by a moderator:
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First, write the tan as sin/cos. Then multiply the equation by cos(bt). Then write the equation in terms of the variable:

[tex]\tau = \sin bt[/tex]

dhris
 
Last edited:
Originally posted by dhris
First, write the tan as sin/cos. Then multiply the equation by cos(bt). Then write the equation in terms of the variable:

[tex]\tau = \sin bt[/tex]

dhris

Thanks for the hint. I actually found this substitution works best…

[tex]\tau = cos(bt)[/tex]

after making the substitution I solved it via a power series method.
 
Originally posted by MathNerd
Thanks for the hint. I actually found this substitution works best…

[tex]\tau = cos(bt)[/tex]

after making the substitution I solved it via a power series method.

But you can solve it exactly if you use the other one!
 

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