# One-loop Feynman diagram

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The evaluation of one-loop Feynman diagrams usually leads to divergent expressions, which are either due to zero-mass particles in the cycle of the diagram (infrared divergence) or due to insufficient falloff of the integrand for high momenta (ultraviolet divergence). The former are usually dealt with by assigning the zero mass particles a small mass λ, evaluating the corresponding expression and then taking the limit $\lambda \to 0$, the latter are dealt with in the renormalization program.
$\Gamma[\phi]=S[\phi]+\frac{1}{2} \mathop{\mathrm{Tr}}{\left[\ln {S^{(2)}[\phi]}\right]+\dots}$