I'm reading about path integrals in Peskin and Schroeder's Introduction to Quantum field theory and there is a few things in the text which I find puzzling. At page 283 in the section about correlation functions we are considering the object (equation 9.15)(adsbygoogle = window.adsbygoogle || []).push({});

[tex] \int D\phi(x) \phi(x_1) \phi(x_2) \exp(i\int_{-T}^T d^4x L(\phi)).[/tex]

Then the author break up the integral into

[tex]\int D\phi_1(\vec x) \int D\phi_2(\vec x) \int_{\phi(x_1^0, \vec x) = \phi_1(\vec x), \phi(x_1^0, \vec x) = \phi_2(\vec x)} D\phi(x).[/tex]

Now that I do not have that much experience with functional integration I was wondering about the justification and reasoning behind this step. Pesking and Schroeder can be a bit brief in places. Does anyone know of any other books with a bit more detail on path integrals which also derives the Feynman rules?

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# Splitting up of functional integral (Peskin and Schroeder)

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