1. See, for example, L. P. Eisenhart,A Treatise on the Differential Geometry of Curves and Surfaces(Ginn and Company, New York, 1909).
2. Cf. Eisenhart, reference 1, pp. 205–206.
3. See, for example, O. Bolza,Vorlesungen über Variationsrechnung(B. G. Teubner, Leipzig, 1933), Chapter 10.