Peskin and Schroeder, equation 2.51

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Hi

I am struggling to justify

D(x-y) \approx e^{-i m t} as t \rightarrow \infty

from

\int dE \sqrt{E^2-m^2} e^{-i E t}.

I thought I might get some insight from discretizing, as

e^{-i m t} \sum_{n=0}^{\infty} \epsilon \sqrt{ n \epsilon ( 2 m + n \epsilon ) } e^{-i n \epsilon t}

but I don't understand how to approximate or take the limit of the sum.

I also tried to work backwards from

G : (E^2-m^2)^{3/2} e^{-i E t}

and replacing extra E's with d/dt's and ended up with a differential equation
for the result,

G = i [ t (d^2/dt^2 + m^2) + 3 d/dt ] D(x-y)

but I am having trouble putting the pieces together to solve it.

Assistance would be greatly appreciated.
 
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