1. W. Yourgrau and S. Mandelstam,Variational Principles in Dynamics and Quantum Theory, 3rd ed. (Pitman & Sons, 1968), pp. 16, 175, 177.
2. W. F. Osgood,Mechanics (MacMillan, 1937), p. 356.
3. M. R. Hestenes,Modern Mathematics for the Engineer (McGraw-Hill, 1956), p. 75.
4. I. M. Gelfand and S. V. Fomin,Calculus of Variations (Prentice Hall, 1963), p. 159.
5. D. R. Smith,Variational Methods in Optimization (Prentice Hall, 1974), p. 195.