Kirsch, Johann Peter (1910). „John XIII“. In Herbermann, Charles G.; Pace, Edward A; Pallen, Condé B.; Shahan, Thomas J.; Wynne, John J. (eds.). The Catholic Encyclopedia. Vol. 8. New York: Encyclopedia Press. p. 427. OCLC13551132.
Carus, Paul (1908 m. sausio mėn.). „Galileo Galilei“. The Open Court. p. 5. Nuoroda tikrinta February 13, 2021.
Kirsch, Johann Peter (1910). „John XIII“. In Herbermann, Charles G.; Pace, Edward A; Pallen, Condé B.; Shahan, Thomas J.; Wynne, John J. (eds.). The Catholic Encyclopedia. Vol. 8. New York: Encyclopedia Press. p. 427. OCLC13551132.