1. M. M. Vainberg,Variational Methods in the Study of Nonlinear Operators, Holden Day, 1964.
2. E. Tonti,Variational formulation of nonlinear differential equations, Bull. Acad. Roy. Belgique, 5^ serie, tome IV (1969), pp. 137–165 (first part), pp. 262–278 (second part).
3. E. Tonti,On the Inverse Problem of the Calculus of Variations, Academic Press (in press).
4. P. M. Morse -H. Feshbach,Methods of Theoretical Physics, vol. I, McGraw-Hill, 1953.
5. C. Lanczos,Linear Differential Operators, Van Nostrand, 1961.