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

  • Context: Graduate 
  • Thread starter Thread starter NoobixCube
  • Start date Start date
  • Tags Tags
    Integral
Click For Summary

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.

NoobixCube
Messages
154
Reaction score
0
Hi all,
I have checked many integral tables and done a lot of working, but I can't seem to derive a solution for:

[tex] \int \frac{1}{(1+a cos(\theta - \phi))^2} d\theta[/tex]

where a is constant.
Any help would be most appreciated
 
Physics news on Phys.org
It would be good for us too see an attempt at the problem...there aren't many people that are kind enough to give you the answer straight up.
 
Possibly Helpful Hint:
1) expand the denominator
2) look up the derivative of [tex]tan^{-1}{(\theta + \phi)}[/tex]

See if you can link the two together...
 
Bet you cant
 
hahahaha :smile: .
nice.
 
yeah I've come across this before in an "applied mathematics" course.
the integral is going to be really big and ugly and involve a lot of hyperbolic arctangents

the real crappy part is that while you're writing out reams of expressions you'll be thinking the whole time that there must be some cute simplification for all this. I mean it really looks like there should be, but there just isn't.

use some software to solve this one. that's why we invented it.
 
oh, by the way the derivative of [tex]tan^{-1}{(\theta + \phi)} = 1/(1 +(\theta - \phi)^2)[/tex]

p.s. sorry, my latex skills are weak
 
Last edited:
tried it in Mathematica it is a nasty solution if it is correct

[tex]-\frac{2 ArcTanh[\frac{(a-1)Tan(\frac{\Theta-\phi}{2})}{\sqrt{a^2-1}}]}{(a^2-1)^{3/2}} + \frac{a * Sin(\Theta-\phi)}{(a^2-1)(1+a*Cos(\Theta-\phi))}[/tex]
 
wow! that's pretty if you ask me
 
  • #10
Thanks for all the input guys, I used MATLAB and Mathematica, both give different solutions...
I have heard rumors that Mathematica can sometimes give wrong answers, can anyone support this claim?
 
  • #11
Oh, by the by, maybe I should have said that a<1 ... this may affect the first term of the solution from Mathematica
 
  • #12
at least a [tex]tan^{-1}{(...)}[/tex] appears in the answer...too bad I can't figure out how or why.
 
  • #13
n0_3sc said:
at least a [tex]tan^{-1}{(...)}[/tex] appears in the answer...too bad I can't figure out how or why.

Can you do a partial fraction expansion?
 
  • #14
NoobixCube said:
Thanks for all the input guys, I used MATLAB and Mathematica, both give different solutions...
I have heard rumors that Mathematica can sometimes give wrong answers, can anyone support this claim?

just differentiate each answer and see which one is the right one
 
  • #15
John Creighto said:
Can you do a partial fraction expansion?

I thought about that too. I think I did attempt it and got stuck somewhere...I can't remember anymore.
 
  • #16
I will opt for differentiating both
thanks
 
  • #17
Would anyone know an easy method to invert this function


[tex] t(\theta)=-\frac{2 ArcTanh[\frac{(a-1)Tan(\frac{\Theta-\phi}{2})}{\sqrt{a^2-1}}]}{(a^2-1)^{3/2}} + \frac{a * Sin(\Theta-\phi)}{(a^2-1)(1+a*Cos(\Theta-\phi))}[/tex]

to
[tex] \theta(t)<br /> [/tex]
?
 
  • #18
Construct a Matrix of [tex]t(\theta)[/tex], do the inverse then the determinant. - i think that's how its done...its been too long.
 
  • #19
what would be the elements of the matrix [tex]t(\theta)[/tex]?
I have never heard of this method before...
 
Last edited:
  • #20
n0_3sc said:
Construct a Matrix of [tex]t(\theta)[/tex], do the inverse then the determinant. - i think that's how its done...its been too long.
? that's for a matrix not for a function
NoobixCube said:
what would be the elements of the matrix [tex]t(\theta)[/tex]?

why do you need to invert that answer? your integrand in the original post is in terms of theta?
 
  • #21
oh wait a sec...
I don't think that method works here because the theta's are in the sin/cos and tan terms...Maybe i'll just be quite and wait for someone smarter to help you.
 
  • #22
ice109 said:
? that's for a matrix not for a function
yeah I remember doing it for multiple equations with multiple variables...
 
  • #23
n0_3sc said:
yeah I remember doing it for multiple equations with multiple variables...

except there's only variable here...

here is my mathematica notebook
 

Attachments

  • #24
ice109 said:
? that's for a matrix not for a function


why do you need to invert that answer? your integrand in the original post is in terms of theta?
Well the original question was a solution to an ODE, but I need the inverted form, so the solution can be of use to me
:-p
 
  • #25
meh use a numerical nonlinear solver.
 
  • #26
how precise do you need it? If you only need an approximation, plot the answer in a (t(theta),theta) coordinatesystem and make a fit.

I doubt you will find an analytic inverse.
 
  • #27
give me values for a and phi and i'll make you a plot
 
  • #28
[tex]a=0.927[/tex]
and
[tex]\phi = 278 degrees[/tex]
 
  • #29
mrandersdk said:
how precise do you need it? If you only need an approximation, plot the answer in a (t(theta),theta) coordinatesystem and make a fit.

I doubt you will find an analytic inverse.

What is the usual course of action if there isn't an analytical inverse?
 
  • #30
NoobixCube said:
What is the usual course of action if there isn't an analytical inverse?

Ignore the problem and move on.
Its not like this is part of a Masters Thesis Research... :-p
 

Similar threads

  • · Replies 8 ·
Replies
8
Views
3K
  • · Replies 12 ·
Replies
12
Views
3K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 3 ·
Replies
3
Views
4K
  • · Replies 29 ·
Replies
29
Views
5K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 3 ·
Replies
3
Views
3K
  • · Replies 16 ·
Replies
16
Views
3K
  • · Replies 27 ·
Replies
27
Views
3K