"Version: 20210204"--Title page verso.Includes bibliographical references.1. The fluctuation-dissipation theorem -- 1.1. Introduction -- 1.2. Linear response theory -- 1.3. Fluctuation-dissipation theorem2. Green's function method -- 2.1. Green's functions and correlations -- 2.2. Spin-waves in ferromagnets -- 2.3. Critical exponents3. Perturbative method for Green's functions -- 3.1. Time inde…