Which principle forms the basis of Variational Perturbation Theory?

Variational Perturbation Theory is based on variational principles, which are fundamental concepts in physics and mathematics. These principles involve finding the function that minimizes or maximizes a certain functional, often representing physical quantities such as energy or action. In the context of Variational Perturbation Theory, variational principles are used to derive equations that describe the behavior of perturbed quantum systems. By minimizing or maximizing the relevant functionals, we can obtain approximate solutions to complex quantum mechanical problems.