Solving Intractable Integral: \frac{1}{(1+a cos(\theta - \phi))^2}

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Discussion Overview

The discussion revolves around the integral \int \frac{1}{(1+a cos(\theta - \phi))^2} d\theta where participants explore methods for solving it, share insights, and discuss the complexity of the solution. The conversation includes attempts at analytical solutions, numerical methods, and the use of software tools like Mathematica and MATLAB.

Discussion Character

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

Main Points Raised

  • One participant expresses difficulty in deriving a solution for the integral and seeks help.
  • Another participant suggests showing an attempt at the problem instead of seeking direct answers.
  • Some participants propose expanding the denominator and looking up the derivative of tan^{-1}{(\theta + \phi)} as a potential approach.
  • A participant mentions that the integral may yield a complex solution involving hyperbolic arctangents.
  • There are claims that software like Mathematica may provide incorrect solutions, with one participant seeking confirmation of this rumor.
  • Another participant notes that the condition a<1 may affect the solution derived from Mathematica.
  • Some participants discuss the possibility of using partial fraction expansion but express uncertainty about its feasibility.
  • One participant suggests differentiating the solutions obtained from different software to verify correctness.
  • There is a discussion about inverting the function derived from the integral, with some participants questioning the method proposed for inversion.
  • Several participants express skepticism about finding an analytical inverse and suggest numerical methods instead.
  • One participant mentions that the problem resembles "Elliptical Integrals" and recommends looking it up for more information.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best method to solve the integral or the reliability of software solutions. Multiple competing views and approaches are presented throughout the discussion.

Contextual Notes

Some participants note the complexity of the integral and the potential for different solutions depending on the method used. There are also references to the limitations of analytical methods and the reliance on numerical solutions.

  • #31
NoobixCube said:
What is the usual course of action if there isn't an analytical inverse?
depends on what you want to use the inverse for?
n0_3sc said:
Ignore the problem and move on.
Its not like this is part of a Masters Thesis Research... :-p

:rolleyes:

what range for theta?
 
Last edited:
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  • #32
say from 0 4pi , or any range that might show the functions periodicy
 
  • #33
Now that I look at your problem - It definitely reminds me of "Elliptical Integrals".
Wiki it and I'm sure you'll find something useful.
 
  • #34
take this as a hint; get mathematica
 

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  • #35
NoobixCube said:
What is the usual course of action if there isn't an analytical inverse?

Apparently its "Get Mathematica".
 
  • #36
If you mean elementary anti derivative, then a) If you have limits of integration, numerical methods, or b) Define it as a new function ! =]
 
  • #37
Thanks for taking the time ice109 :smile:
 
  • #38
You can solve this integral by considering it after the simple substitution:

\theta- \phi=t

as:

\int\frac{dt}{(1+acos(t))^2}

Now use the substitution:

1+acos(t)=\frac{1-a^2}{1-acos(t)}

You will easily arrive at the solution.

Take a look at an older post of me where I explain this substitution a bit more:

https://www.physicsforums.com/showthread.php?t=204639

Hope this helps.
 

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