# Using the method of steepest descent

1. Oct 29, 2005

### Hoplite

I have the question,
$$\int_{-\pi/2}^{\frac{\pi}{2}} e^{-ilk}cos^n kdk$$
It says, "Set t=ik". So,
$$-i\int_{-i\pi/2}^{i\pi/2}e^{-lt} cosh^n tdt$$
But then it says, "Use the method of steepest descent to show that as n $$\rightarrow \infty$$ with r = l/n."
I'm supposed to get:
$$\sim \sqrt{\frac{2\pi}{n(1-r^2)} }exp(-\frac{1}{2}n[r\log{\frac{1+r}{1-r}}+log(1-r^2)])$$
If the equation were of the form, $$\int e^{ilP(t)}Q(t)dt$$, 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. Oct 29, 2005

### Tide

HINT: Write $\cosh ^n x = e^{\ln \cosh^n x}$

3. Oct 29, 2005

### Hoplite

Thanks, Tide. That question was killing me.

4. Jul 20, 2011