Zlatarski gives another chronology, pp. 141-142. See also GIBI, V, p. 16
Irechek, K., History of the Bulgarians, p. 661; Zlatarski, p. 859; Kosev D. and Co., History of Bulgaria, Edition of the Bulgarian Academy of Science, Sofia, 1981, vol. 2, p. 472
nbu.bg
see Gyuzelev, p. 114; Besevliev V., Die protobulgarische Periode der bulgarischen Geschichte, Verlag Adolf M. Hakkert, Amsterdam, 1980, p. 349; Cholova, Ts.Archived 2010-11-26 at the Wayback Machine, On the co-ruler of the monarch of the First Bulgarian State in the collection Bulgaria 1300. Institutions and state tradition, Bulgarian Historical Society, Sofia, 1982, vol. II, pp. 209-214, and also: Presian Inscription
web.archive.org
see Gyuzelev, p. 114; Besevliev V., Die protobulgarische Periode der bulgarischen Geschichte, Verlag Adolf M. Hakkert, Amsterdam, 1980, p. 349; Cholova, Ts.Archived 2010-11-26 at the Wayback Machine, On the co-ruler of the monarch of the First Bulgarian State in the collection Bulgaria 1300. Institutions and state tradition, Bulgarian Historical Society, Sofia, 1982, vol. II, pp. 209-214, and also: Presian Inscription