1. N. N. Bogolyubov, Ukr. Matem. Zh.,2, 3 (1950); see also: Selected Works [in Russian], Vol. 2, Kiev (1970).
2. S. V. Tyablikov, Zh. Eksp. Teor, Fiz.,21, 377 (1956).
3. E. P. Solodovnikova, A. N. Tavkhelidze, and O. A. Khrustalev, Teor. Mat. Fiz.,10, 162 (1972).
4. E; P. Solodovnikova, A. N. Tavkhelidze, and O. A. Khrustalev, Preprint JINR E2-5976 [in English], Dubna (1971).
5. L. P. Eisenhart, Continuous Groups of Transformations [Russian translation], IL (1947).