В. М. Бухштабер, А. П. Веселов, “Топограф Конвея, PGL2(Z)-динамика и двузначные группы”, УМН, 74:3(447) (2019), 17–62; V. M. Buchstaber, A. P. Veselov, “Conway topograph, PGL2(ZZ)-dynamics and two-valued groups”, Russian Math. Surveys, 74:3 (2019), 387–430
V. M. Buchstaber, “n-valued groups: theory and applications”, Mosc. Math. J., 6:1 (2006), 57–84
Alexander N. Kholodov, “Formal coalgebras and applications”, Journal of Pure and Applied Algebra, 85:3 (1993), 271