1. Encyclopaedia of Mathematical Sciences. 41;A. L. Onishchik,1994
2. A. T. Fomenko and D. B. Fuks, A course in homotopic topology (Nauka, Moscow, 1989) [in Russian].
3. A. Hatcher, Algebraic topology (Cambridge University Press, Cambridge, 2002).
4. M. Mimura and H. Toda, Topology of Lie Groups. I, II (Amer. Math. Soc., Providence, RI, 1991).
5. V. L. Popov, Underlying Varieties and Group Structures, arXiv: 2105.12861.v1, 2021.