↑ George L. Williams, Papal Genealogy: The Families and Descendants of the Popes, (McFarland & Company Inc., 1998), 15. ↑ Gregorovius, Ferdinand (1903), The History of the City of Rome in the Middle Ages, III (2nd ed.), London: George Bell & Sons, p. 254, retrieved 2008-01-06 ↑ Baxmann, Rudolf (1869), Die Politik der Päpste von Gregor I. bis Gregor VII, II, Elberfeld, pp. 58–125. ↑ Platina, Bartolomeo (1479), The Lives of the Popes From The Time Of Our Saviour Jesus Christ to the Accession of Gregory VII, I, London: Griffith Farran & Co., pp. 248–249, retrieved 2013-04-25 ↑ "Sergius at once declared the ordinations conferred by Formosus null; but that he put his two predecessors to death, and by illicit relations with Marozia had a son, who was afterwards John XI, must be regarded as highly doubtful.