↑Richards, Jeffrey. The popes and the papacy in the early Middle Ages, 476-752. Londres; Boston: Routledge & Kegan Paul, 1979, p. 207. ISBN 9780710000989.
↑Deliyannis, Deborah Mauskopf. The book of pontiffs of the Church of Ravenna. Washington, DC: Catholic University of America Press, 2004, p. 238. ISBN 9780813213583.