Schwinger parametrization

The Schwinger parametrization is a method for evaluating loop integrals in Feynman diagrams with more than zero 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 that this evaluation is easy

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

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