Angold 1997, p. 122; Stephenson 2000, pp. 99–100. (EN) Michael Angold, The Byzantine Empire, 1025–1204: A Political History, 2ª ed., 1997, Longman, ISBN0-582-29468-1. (EN) Paul Stephenson, Byzantium's Balkan Frontier: A Political Study of the Northern Balkans, 900–1204, Cambridge, New York, Cambridge University Press, 2000, ISBN0-521-77017-3, OCLC52413047.
Angold 1997, pp. 121–122; Stephenson 2000, pp. 98, 100. (EN) Michael Angold, The Byzantine Empire, 1025–1204: A Political History, 2ª ed., 1997, Longman, ISBN0-582-29468-1. (EN) Paul Stephenson, Byzantium's Balkan Frontier: A Political Study of the Northern Balkans, 900–1204, Cambridge, New York, Cambridge University Press, 2000, ISBN0-521-77017-3, OCLC52413047.