Dynamical correlations

In this work we review the theoretical methods implemented in the last decades to account for the dynamical properties of condensed systems, as probed by suitable time correlation functions and/or by the associated frequency spectra. A host of approaches, which make use of auxiliary quantities such as Greenʹs and relaxation functions, is discussed in detail. Specifically, these techniques are applied to unravel the dynamics of Heisenberg ferromagnets and of simple fluids, respectively taken as representative of quantum and of classical many-body systems.