Single-particle response of finite nuclei

After a collection of relevant definitions, the natural-orbital basis is used to derive an equation for the spectroscopic amplitudes entering the hole spectral density. The solutions for the discrete and the continuous spectrum are then discussed. The application to (e,e'p) and (#betta#,p) reactions gives the possibility of improving the theoretical description of such knock-out processes. The conclusions are drawn in the final section. (orig./HSI)