1. A. D. Sakharov, Dokl. Akad. Nauk SSSR,177, 1, 70 (1967).
2. A. D. Sakharov, in: Gravitation and Field Theory (Collection of Preprints of the Institute of Applied Mathematics), October (1967).
3. L. D. Landau and I. Ya. Pomeranchuk, Dokl. Akad. Nauk SSSR,102, 489 (1955); E. S. Fradkin Dokl. Akad. Nauk SSSR,98 (1954);100, 897 (1955); Ya. B. Zel'dovich, ZhÉTF Pis'ma Red.,6, 1233 (1967).
4. H. P. McKean, Jr., and I. M. Singer, J. Differ. Geom.,1, 43 (1967).
5. Ya. B. Zel'dovich, Usp. Fiz. Nauk,95, 209 (1968); oral communication in 1967.