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Homework Help: Incomplete solution of integral

  1. Jan 8, 2013 #1
    The task is to integrate dθ/(k-cosθ) from -pi to pi.

    This is the case - I have solved this and also seen the teacher solve this on the board but I am not satisfied with the method and answer. We used the method of resdues.

    This is how my instructor would solve it:




    x^2+y^2=1 implies Q(x,y)≠0 iff |k| is greater than 1.

    -1≤k≤1 implies that Q(x,y)=0 iff x=k.

    Indeed this seems legitimate.

    He then evaluates the residues for |k| greater than 1 only.

    However, when k is not greater than one, the poles lie on the unit circle in the finite/complex plane. In this case the residue method cannot be used and the answer is therefore not complete. What is the solution of this integral without the assumption that |k| is greater than 1?

  2. jcsd
  3. Jan 8, 2013 #2


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    Staff: Mentor

    Assuming real k: with ##|k| \leq 1##, your function is not well-defined everywhere in the integration range, and you need a different way to write the problem.
  4. Jan 16, 2013 #3
    Yeah, your assumtion is correct. Well, that is how the problem was stated. All you are given is the integral, which I have presented in words instead of using the integral sign. Thus, how would I write it in a different way to make it possible to solve?
  5. Jan 16, 2013 #4


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    It depends on the integral you want to calculate, and the reason why you want to do that.

    Something like
    $$\lim_{\epsilon \to 0} \left( \int_{-\pi}^{\arccos(k)-\epsilon} \frac{d\theta}{k-cos(\theta)} + \int_{\arccos(k)+\epsilon}^{\pi} \frac{d\theta}{k-cos(\theta)} \right)$$
    might be possible to evaluate.
  6. Jan 20, 2013 #5
    Ah, of course! That's Cauchy's principal value of the integral. :)
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