1. *LM. Gel’fand, S.V. Fomin: Calculus of Variations (Fizmatgiz, Moscow 1961) [Prentice-Hall, Englewood Cliffs, NJ 1963]
2. D. Laugwitz: Differential and Riemannian Geometry (Academic, New York 1965)
3. *P.K. Rashevskii [Raschewskii]: Riemannsche Geometrie und Tensor analysis, 3rd ed. (Nauka, Moscow 1967) [1st ed., VEB Deutscher Verlag der Wissenschaften, Berlin 1959]
4. H.W. Guggenheimer: Differential Geometry (Dover, New York 1977)
5. J.B. Keller: “A Geometrical Theory of Diffraction”, in Calculus of Variations and Its Applications, ed. by L.M. Graves (McGraw-Hill, New York 1958) pp. 27–52