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Works about Pope GregoryEdit
- "Gregory IX." in the Encyclopædia Britannica, 9th ed. 1880.
- "Gregory IX." in the Encyclopædia Britannica, 11th ed. 1911.
- "Pope Gregory IX," in Catholic Encyclopedia, (ed.) by Charles G. Herbermann and others, New York: The Encyclopaedia Press (1913)
Some or all works by this author were published before January 1, 1928, and are in the public domain worldwide because the author died at least 100 years ago. Translations or editions published later may be copyrighted. Posthumous works may be copyrighted based on how long they have been published in certain countries and areas.