1. S. Z. Shefel', ?Surfaces in Euclidean space,? in: Mathematical Analysis and Related Questions of Mathematics [in Russian], Nauka, Novosibirsk (1978), pp. 297?318.
2. G. S. Shefel', ?Groups of transformations of Euclidean space,? Sib. Mat. Zh.,26, No. 3, 197?215 (1985).
3. S. Z. Shefel', ?Geometric properties of embedded manifolds,? Sib. Mat. Zh.,26, No. 1, 171?188 (1985).
4. V. I. Arnol'd, A. N. Varchenko, and S. M. Gussein-Zade, Singularities of Differentiable Maps [in Russian], Vol. 1, Nauka, Moscow (1982).
5. A. Z. Petrov, Einstein Spaces [in Russian], Fizmatgiz, Moscow (1961).