1. N.J. Akhiezer, The calculus of variations, Biaisdell Publ. Co., New York, 1962.
2. N.I. Akhiezer and I.M. Glazman, Theory of linear operators in Hilbert Space, vol. I & II, Ungar Publishing Company, New York, 1963, translated from Russian by M. Nestell.
3. A. Alexiewicz and W. Orlicz, On differentials in Banach Spaces, Ann. Soc. Polon. Math., 25, (1952), p. 95–99.
4. A.M. Arthurs, Complementary Variational Principles, Clarendon Press, Oxford University, Oxford, 1971.
5. A.M. Arthurs, A note on Komkov’s class of boundary value problems and associated variational principles, J. Math. Analysis & Applications, 33, (1971), p. 402–407.