Figure 1
From: Perturbation theory of a superconducting 0 − π impurity quantum phase transition
Diagrammatic representation of the first two orders of the perturbation expansion in the Coulomb interaction for the normal (top) and anomalous (bottom) parts of the self-energy.
The wavy line represents the Coulomb interaction and the lines with single (double) arrow represent the normal (anomalous) propagators according to equation (4).
