¹¹ The people who watch over such matters (and there are such people) would probably insist that Erdös numbers only apply to people who have co-written formal, mathematical papers, and not, for example, textbooks and introductory works. Given Erdös’s love for the “epsilons” (the conventional symbol for a small, positive quantity and the term he routinely used to designate children), we feel confident that anyone who devotes their life to teaching mathematics to the young is a friend of Paul.