  We propose a nonperturbative renormalization-group (NPRG) approach to fermion
systems in the two-particle-irreducible (2PI) effective action formalism, based
on an exact RG equation for the Luttinger-Ward functional. This approach
enables us to describe phases with spontaneously broken symmetries while
satisfying the Mermin-Wagner theorem. We show that it is possible to choose the
Hartree-Fock--RPA theory as initial condition of the RG flow and argue that the
2PI-NPRG is not restricted to the weak-coupling limit. An expansion of the
Luttinger-Ward functional about the minimum of the 2PI effective action
including only the two-particle 2PI vertex leads to nontrivial RG equations
where interactions between fermions and collective excitations naturally
emerge.
