O'Connor, John J.; Robertson, Edmund F. (1998). „Charles Babbage“. MacTutor History of Mathematics archive. School of Mathematics and Statistics, University of St Andrews, Scotland. Suarchyvuotas originalas 2006-06-16. Nuoroda tikrinta 2006-06-14.
O'Connor, John J.; Robertson, Edmund F. (1998). „Charles Babbage“. MacTutor History of Mathematics archive. School of Mathematics and Statistics, University of St Andrews, Scotland. Suarchyvuotas originalas 2006-06-16. Nuoroda tikrinta 2006-06-14.