1. L.D. Landau and E.M. Lifshitz, Mechanics, 131; H. Goldstein, Classical Mechanics, (Pergamon Press, Oxford, 1960), p. 131.
2. H. Goldstein, Classical Mechanics, (Addison-Wesley, Reading, MA, 1980), p. 341.
3. A. Kolmogoroff, I. Petrovsky, N. Piscounoff, Bull. Univ. Moscow, Ser. Int., Sec. A 1, 1 (1937), p. 5.
4. D.G. Aronson and H.F. Weinberger, Partial Differential Equations and Related Topics, Vol. 446 of Lecture Notes in Mathematics (Springer, New York, 1975), p. 5. Adv. Math.
30, 33 (1978).
5. D.G. Aronson and H.F. Weinberger, Adv. Math.
30, 33 (1978).