1. A. Yu. Ol’shanskii, “Periodic factor of hyperbolic groups,” Math. USSR-Sb. 72 (2), 519–541 (1992).
2. A. Yu. Ol’shanskii, Geometry of Defining Relations in Groups (Nauka, Moscow, 1989) [in Russian].
3. I. S. Ashmanov and A. Yu. Ol’shanskiǐ, “Abelian and central extensions of aspherical groups,” Soviet Math. (Iz. VUZ) 29 (11), 65–82 (1985).
4. S. I. Adian, “On some torsion-free groups,” Math. USSR-Izv. 5 (3), 475–484 (1971).
5. S. I. Adyan, The Burnside Problem and Identities in Groups (Nauka, Moscow, 1975) [in Russian].