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Contour intg.

  1. Sep 2, 2009 #1
    1. The problem statement, all variables and given/known data

    evaluate the contour integral

    int.cont. Im(z) dz

    where cont. is the path from the point z_o = 1 + i to the point z_1 = 3 + 9i

    along the parabolic curve y = x^2. sketch the contour

    2. Relevant equations

    int.[a to b] z(t)z'(t) dt

    3. The attempt at a solution

    I parametrized z(t) = t + t^2*i and runs 1 <= t <= 3

    z'(t) = 1 + 2ti

    Here is where I get confused

    I know Im(z(t)) = t^2*i, but do I also have to do Im(z'(t)) = 2ti?

    so im unsure whether to evaluate

    int [from 1 to 3] Im(z(t))*Im(z'(t)) dt


    int [from 1 to 3] Im(z(t))*(z'(t)) dt

    Also, since we are integrating the imaginary part only, how does the sketch differ from a contour integral of lets say z(t)dt

  2. jcsd
  3. Sep 2, 2009 #2


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    Gold Member

    I think you are being confused by the fact that the function you are integrating takes the imaginary part of its argument. Think about how you would solve the problem generally, look at f(z) instead of Im(z). Im(z) is just a special case.

    If you were integrating f(z), would you then also take f(z') (If that makes sense)?

    Also, your equation under 'relevant equations' is not quite right. The equation
    does not take into account that you are looking at the imaginary part. How should this equation look for a general f(z)?
  4. Sep 2, 2009 #3

    I 've a question here:
    sin (z) / (z^15)

    How to find singularities; locale etc?
    Related to this theory?
  5. Sep 3, 2009 #4
    for general f(z)

    isnt it

    int [ a to b ] [u(t) + iv(t)] dt


    int [a to b] u(t) dt + i* int [ a to b] v(t) dt

    then im just solving

    i* int [ a to b] v(t) z'(t) dt


  6. Sep 3, 2009 #5


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    How are you defining u(t) and v(t)? Is it f(z(t)) = u(z(t)) + i*v(z(t)), or z(t) = u(t) + i*v(t) ? Depending on which one you mean, your last equation may be correct. Your first two equations, though, are missing something. How do you convert an integral 'dz' to an integral 'dt'?
  7. Sep 3, 2009 #6
    please ignore the first two equations as they were pertaining to f(t) not f(z(t)). I know u have to add the derivative of z(t).

    in the last equation,

    i* int [ a to b] v(t) z'(t) dt

    u(t) and v(t) are defined in accord to

    f(z(t)) = u(z(t)) + i*v(z(t))

    my final integral looks like this

    int [from 1 to 3] (t^2*i)(1 + 2ti) dt

    is this correct? thanks for the help!
    Last edited: Sep 3, 2009
  8. Sep 3, 2009 #7


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    You have the idea exactly right. There is a minor problem with your interpretation of Im(z). The way it is defined is that for z = x + i*y (x and y real), Im(z) = y, not Im(z) = i*y. You are treating it like it is the second case.
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