R. Grousset: The Empire of the Steppes: A History of Central Asia. Rutgers University Press, 1988, S. 585, (online): “[…] Khagan was the old Juan-juan – therefore Mongol – title, afterward adopted by the kings of the T'u-chüeh Turks, destroyers of and successors to the Juan-juan empire […]”
iranicaonline.org
K. H. Menges: Altaic people. In: Encyclopaedia Iranica. V, S. 908–912, Online Edition (online): “[…] The Avars […] are without doubt the first Mongolian group to be historically attested […] They were the first to use the title [Ḫān] for the supreme ruler […]”
