1-particle irreducible correlation function

From Physics wiki

We denote the sum of 1-particle irreducible diagrams by

,

where it should be understood that external propagators are not to be included. We can express the self-energy as follows

or, written as the Dyson equation

,

which is equivalent to

.

Recalling the Källén-Lehmann spectral representation, we see that represents a shift in mass.