We discuss Ward identities for strongly interacting fermion systems described by Eliashberg-type theories. We show that Ward identities are not in conflict with the Migdal theorem and derive diagrammatically a Ward identity for a charge vertex both in a Fermi liquid and when a Fermi liquid is destroyed at quantum criticality. We argue that a Ward identity for the spin vertex cannot be obtained within the Eliashberg theory.

• Received 1 November 2004

DOI:https://doi.org/10.1103/PhysRevB.72.085113