This text presents extended separation, comparison, and oscillation theorems that replace classical analysis of Legendre, Jacobi, and Hilbert. Its analysis of related quadratic functionals shows how critical extremals can substitute for minimizing extremals. Additional topics include focal conditions and Sturm-like theorems, general boundary conditions, and root theory prestructures. 1973 edition. Reprint of the John Wiley & Sons, New York, 1973 edition.
Here's a sample of other books in this Dover category
Variational Principles by B. L. Moiseiwitsch This text shows how variational principles are used to determine the discrete eigenvalues for stationary state problems and to illustrate how to find the values of quantities that arise in the theory of scattering. 1966 edition.
Variational Methods in Optimization by Donald R. Smith Highly readable text elucidates applications of the chain rule of differentiation, integration by parts, parametric curves, line integrals, double integrals, and elementary differential equations. 1974 edition.
