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 holeone particle! response propagator. The aim is to deal with particleparticle ~and hole-hole! as well as particle-hole collectivity simultaneously. This is achieved on a TammDancoff 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 RPAwhich was sometimes applied in the past, has serious unsolved problems, some of which are discussed in detail.