V. Jaksic, E. Kritchevski, C.-A. Pillet Mathematical Theory of the Wigner-Weisskopf Atom (1244K, pdf) ABSTRACT. These notes provide an introduction to the spectral theory of the Friedrichs model. This model is used in quantum physics to describe the coupling of a discrete set of energy levels to a continuum of states. We restrict ourselves to the simplest possible case of a single energy level coupled to a continuum, the so-called Wigner-Weisskopf atom. We discuss both, perturbative and non-perturbative aspects. We also consider the fermionic second quantization of this model and discuss its non-equilibrium thermodynamics: steady states, steady currents and entopy production.