Hole spectral function and two-particle–one-hole response propagator

The fragmentation of one-nucleon knock-out strength at low energies is considered from the viewpoint of the two particle-one hole (and two hole- one particle) response propagator. The aim is to deal with particle-particle (and hole-hole) as well as particle-hole collectivity simultaneously. This is achieved on a Tamm-Dancoff level by the so-called Faddeev approximation of the 2p1h propagator. Results of this approach illustrate the relevance of this consistent treatment of both particle-particle (hole-hole) and particle-hole collectivity. A further extension, within the framework of 2p1h RPA which was sometimes applied in the past, has serious unsolved problems, some of which are discussed in detail. {} 1996 The American Physical Society.