Koibaev V. A., “Subgroups of the group ${\rm GL}(2,Q)$ that contain a nonsplittable maximal torus”, Dokl. Akad. Nauk SSSR, 312:1 (1990), 36–38; Soviet Math. Dokl., 41:3 (1990), 414–416 (1991)
Borevich Z. I., Koibaev V. A., Chan Ngok Khoi, “Lattices of subgroups in ${\rm GL}(2,Q)$ that contain a nonsplittable torus”, Voprosy Teor. Predstav. Algebr i Grupp. 1, Zap. Nauchn. Sem. Leningrad. Otdel. Mat. Inst. Steklov. (LOMI), 191, 1991, 24–43; J. Soviet Math., 63:6 (1993), 622–633
; Soviet Math. Dokl., 41:3 (1990), 
