Папа Гргур IV — разлика између измена

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* {{Cite book |ref= harv|last=Hughes|first=Philip|title=History of the Church|volume=II|year=1948|id=|pages=183}}
* Levillain, Philippe, ''The Papacy: Gaius-Proxies'', Routledge (2002)
* {{cite book|ref= harv|last=Mann|first=Horace K.|title=The Lives of the Popes in the Early Middle Ages, Vol. II: The Popes During the Carolingian Empire, 795-858|year=1906}}
* DeCormenin, Louis Marie; Gihon, James L., ''A Complete History of the Popes of Rome, from Saint Peter, the First Bishop to Pius the Ninth'' (1857)