The Development of the Action Principle: A Didactic History from Euler-Lagrange to Schwinger

Short Historical Introduction.-  Curva Elastica.- The Curva Elastica, a Curve of Least Energy.- From Euler to Lagrange.- Laplace and the Capillary - 1807.- A Final Application in Elasticity with Jacobi Elliptic Functions.- Short List of Jacobi Elliptic Functions and Constants Used in Chapter 5.- Variational Methods for Periodic Motions; Mathieu Functions.-  Lagrangian for Isentropic Irrotational Flow.- Action Principle in Classical Electrodynamics.- The Two Giants in Gravity: Einstein and Hilbert.- The Quantum Action Principle.- The Action Principle in Quantum Field Theory.- Quantum Field Theory on Space-Like Hypersurfaces.- Lagrangian Formulation of Gauge Theories.- Effective Actions (Lagrangians) in Quantum Field Theory.-  Modified Photon Propagation Function, Source Theory.