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## Main Question or Discussion Point

would someone mind opening his/her beloved P&S and help me out..

on p.20 it says if we expand the classical Klein-Gordon field in Fourier (momentum) space, we arrive at equation 2.21...why is that?

gracias

on p.20 it says if we expand the classical Klein-Gordon field in Fourier (momentum) space, we arrive at equation 2.21...why is that?

gracias