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In Srednicki's chapter on cross sections, when he calculating the probability of a particular process from the overlap [tex] \langle f\mid i\rangle[/tex] he comes across:

[tex] [(2\pi)^4\delta^4(k_{in}-k_{out})]^2 [/tex]

He states this is can be equated as follows:

[tex] [(2\pi)^4\delta^4(k_{in}-k_{out})]^2= (2\pi)^4\delta^4(k_{in}-k_{out})\times (2\pi)^4\delta^4(0) [/tex]

I presume from the rest of the text that he is evaluating this as if it was being integrated over $k$, but I still can't see where this comes from. I've tried google but all I seem to find is alot of discussion about people not being sure if the square of the delta function is even well defined.

Thanks

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# Squaring the delta function in QFT(Srendicki ch11)

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