P. Partner, The Lands of St. Peter: The Papal State in the Middle Ages and the Early Renaissance, Berkeley - Los Angeles, 1972, pp. 18–19, W. Ullmann, The Growth of Papal Government in the Middle Ages: A Study in the Ideological Relation of Clerical to Lay Power, Londen - New York, 1970³ (= 2010), pp. 54–55.
