^Iben Fonnesberg-Schmidt, The Popes and the Baltic Crusades: 1147–1254 (Brill, 2007), pp. 197–198.
^Jessalynn Bird, Edward Peters and James M. Powell (eds.), Crusade and Christendom: Annotated Documents in Translation from Innocent III to the Fall of Acre, 1187-1291 (University of Pennsylvania Press, 2013), p. 269.