1. A. M. Butlerov, Selected Works on Organic Chemistry [in Russian] ( Moscow, Izd. AN SSSR, 1951 ) p. 72.
2. A. M. Butlerov, Introduction to a Complete Study of Organic Chemistry [in Russian] (Kazan’, 1864 ) p. 672.
3. V. V. Markovnikov, Data on the Intereffect of Atoms in Chemical Compounds [in Russian] (Kazan’, 1869 ).
4. J. W. Baker, Hyperconjugation (Oxford, 1952 ).
5. F. Becker, Hyperconjugation, Fortschr. Chem. Forsch. 3, 187 (1955).