Noble, Thomas (1995), McKitterick, Rosamond (ed.), "The papacy in the eighth and ninth centuries", The New Cambridge Medieval History: Volume 2: c.700–c.900, The New Cambridge Medieval History, vol. 2, Cambridge: Cambridge University Press, pp. 561–586, ISBN978-0-521-36292-4, retrieved 23 October 2022
Cf., for example, Duchesne, Les premiers temps de l'état pontifical (Paris, 1904), p. 301; and the detailed account in the old Catholic EncyclopediaPope Formosus