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Computing integrals on the half line

  1. Feb 16, 2012 #1

    hunt_mat

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    Hi,

    In my fluids work I have come to integrals of the type:
    [tex]
    \int_{0}^{\infty}\frac{e^{ikx}}{ak^{2}+bk+c}dk
    [/tex]
    I was thinking of evaluating this via residue calculus but I can't think of the right contour, any suggestions?

    Mat
     
    Last edited: Feb 16, 2012
  2. jcsd
  3. Feb 17, 2012 #2
    Try the punctured disc with boundary ##C_{\epsilon}\cup[\epsilon,R]\cup C_R.##
     

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  4. Feb 17, 2012 #3
    Notice that [itex]\vert e^{i \, k \, x} \vert = e^{-x \, \mathrm{Im}k}[/itex]. This means that the integral would diverge when we take the circle at infinity on the lower (upper) semicircle for positive (negative) x.
     
  5. Feb 17, 2012 #4
    Notice that the inverse Fourier transform of the Heaviside step function:
    [tex]
    \int_{-\infty}^{\infty}{\frac{d k}{2\pi} \, \theta(k) \, e^{i \, k \, x}} = -\frac{1}{2\pi \, i \, x}, \ \mathrm{Im}x > 0
    [/tex]
    Thus, we may represent the Heaviside step function as:
    [tex]
    \theta(k) = -\frac{1}{2\pi \, i} \, {d t \, \frac{e^{-i \, k \, t}{t + i \, \eta}}, \ \eta \rightarrow +0
    [/tex]

    Why do we need it? Because your integral goes to:
    [tex]
    \int_{-\infty}^{\infty}{f(k) \, e^{i \, k \, x} \, \theta(k)}
    [/tex]
    If you substitute the integral representation for the step function and change the order of integration, you should get:
    [tex]
    -\frac{1}{2\pi \, i} \, \int_{-\infty}^{\infty}{\frac{d t}{t + i \, \eta} \, \int_{-\infty}{\infty}{f(k) \, e^{i \, k \, (x - t)}}}
    [/tex]
    Now, you may use the residue theorem for the integral over k, but you need to close the contour in different half-planes, depending on whetgher [itex]x > t[/itex] or [itex]x < t[/itex]. The remaining integral over t is again over the whole real line, but , due to the above conditions, should be split into [itex]-\infty[/itex] to x, and from x to [itex]\infty]. Then, making a sub
     
  6. Feb 20, 2012 #5

    hunt_mat

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    I should point out that [itex]x\in\mathbb{R}[/itex]

    Dick, can you explain the substitution again, I don't quite get what you're doing here and you still haven't mentioned the contour you're integrating over.
     
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