Beiträge der irreduziblen Feynman-Diagramme zur Energieabhängigkeit der Quasiteilchen-Streuamplitude
The evaluation of the Feynman diagrams contributing to the quasiparticle scattering amplitude Γω exhibits for special momenta the impossibility of expanding this function into a power series in terms of ω at ω=0 because of the existence of a branch cut in the complex energy plane. The value of the discontinuity across the cut is proportional to ¦ω¦.