1. J.-P. M. Lebrun:Lett. Nuovo Cimento,31, 390 (1980);34, 413 (1982);Nuovo Cimento A,99, 211 (1988).
2. J.-P. M. Lebrun:Lett. Nuovo Cimento,39, 257 (1984).
3. J.-P. M. Lebrun:Nuovo Cimento A,101, 515 (1989).
4. General references on differential equations:A. R. Forsyth:A Theory of Differential Equations (3 vols.) (Cambridge University, reprint by Dover, N.Y.);E. Goursat:Analyse mathématique, Vol.2, part 2 (differential equations) (Cambridge University, reprint by Dover, N. Y.);V. Nemitzkii andV. Stepanov:Qualitative Theory of Differential Equations (Princeton University Press, Princeton, N. J., 1960);R. Bellman:Stability Theory of Differential Equations (London, 1953, reprint by Dover, New York, N.Y.).