## Integration question in Peskin and Schroeder

Hi all, I'm stuck with proving the last step of (2.51) in Peskin and Schroeder:
\begin{align} D(x-y) &= \frac{1}{4\pi^2}\int^\infty_m dE \sqrt{E^2 - m^2}e^{-iEt}\\ & \approx_{t \to \infty}\ \ e^{-imt}\end{align}

I've read on another post that the solution is to use the method of stationary phase, but I do not see how this applies, as $E$ is not a rapidly oscillating function...?

Thoughts appreciated,

Ianhoolihan

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 Recognitions: Science Advisor You didn't say which "other thread" you read, but my post #13 in this thread might shed a little more light... http://www.physicsforums.com/showthread.php?t=233950
 Thanks strangerep, The thread I referred to was http://www.physicsforums.com/showthread.php?t=424778. Your post does make it clearer, in that it stems from a limit of the exact solution (Bessel function). I will look through the details soon. Cheers.
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