The Euclidean Path Integral
In this chapter, we consider the Euclidean path integral calculation for some model examples: namely the quantum-mechanical problems of finding the ground state energy and the wave function. The reason for this choice is that the method of solution of these problems can be generalised directly to QFT. The key feature of the approach to be considered is the use of nontrivial classical trajectories called instantons to evaluate the Euclidean PI. We will go into detail in order to explore this calculation method. It will be the final time in this book that we compare these results with those obtained by the standard methods of quantum mechanics.