Free Energy and Semi-classical Propagator

This lecture covers the concept of free energy as a sum of connected diagrams and the semi-classical propagator, emphasizing the calculation of coefficients and the general approximation methods used in quantum physics. The instructor discusses the perturbative theory and the semiclassical approximation when the wavelength of a particle is much shorter than the typical length over which the potential varies, providing insights into the geometric optics limit of wave propagation. The lecture also delves into the computation of propagators and the importance of vertices in graph theory.