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Homework Help: Using the method of steepest descent

  1. Oct 29, 2005 #1
    I have the question,
    [tex] \int_{-\pi/2}^{\frac{\pi}{2}} e^{-ilk}cos^n kdk[/tex]
    It says, "Set t=ik". So,
    [tex]-i\int_{-i\pi/2}^{i\pi/2}e^{-lt} cosh^n tdt[/tex]
    But then it says, "Use the method of steepest descent to show that as n [tex]\rightarrow \infty[/tex] with r = l/n."
    I'm supposed to get:
    [tex]\sim \sqrt{\frac{2\pi}{n(1-r^2)} }exp(-\frac{1}{2}n[r\log{\frac{1+r}{1-r}}+log(1-r^2)])[/tex]
    If the equation were of the form, [tex]\int e^{ilP(t)}Q(t)dt[/tex], I know how to use the method of steepest descent. I'd find a point z where P'(t)=0 and expand P(t) around that point using a Taylor series expansion getting, P(t)=P(z)+0.5P''(z)(t-z)^2, and then I'd replace t with z+ix and it would all come out from there. But I have no idea how to use the method of steepest descent when P(t)=t and i has been removed from the exponential.
    Last edited: Oct 29, 2005
  2. jcsd
  3. Oct 29, 2005 #2


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    HINT: Write [itex]\cosh ^n x = e^{\ln \cosh^n x}[/itex]
  4. Oct 29, 2005 #3
    :smile: Thanks, Tide. That question was killing me.
  5. Jul 20, 2011 #4
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