Schwinger parametrization

Schwinger parametrization is a technique for evaluating loop integrals which arise from Feynman diagrams with one or more loops.

Using the well-known observation that

$\frac{1}{A^n}=\frac{1}{(n-1)!}\int^\infty_0 du \, u^{n-1}e^{-uA},$

Schwinger noticed, how one may simplify the integral:

$\int \frac{dp}{A(p)^n}=\int dp \int^\infty_0 du \, u^{n-1}e^{-uA(p)}=\int^\infty_0 du \, u^{n-1} \int dp \, e^{-uA(p)}.$

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