Mann, Horace Kinder (1912), "Sergius III", The Catholic Encyclopedia, New York: Robert Appleton Company, XIII, retrieved 2008-01-06 ↑ Poole, Reginald L. (1917), "Benedict IX and Gregory VI", Proceedings of the British Academy, 8: 230. ↑ Gfrörer, August Friedrich, Allgemeine Kirchengeschichte, III, Stuttgart: A. Krabbe, pp. 1133–1275, retrieved 2008-01-06 ↑ Kenrick, Francis Patrick (1855), The Primacy of the Apostolic See Vindicated, Baltimore: John Murphy & Co., p. 418, retrieved 2008-01-06 1 2 One or more of the preceding sentences incorporates text from a publication now in the public domain: Herbermann, Charles, ed.