Hungarian mathematician Paul Erdös was immensely prolific — he published about 1,500 articles in his lifetime. His influence is so great that his colleagues have taken to assigning “Erdös numbers” to one another. Erdös himself gets an Erdös number of 0; his direct collaborators get a 1; anyone who collaborates with them gets a 2, and so on.

Those in the first rank include many of the world’s top mathematicians, but there’s one standout: Hank Aaron. The Baseball Hall of Famer once signed a baseball with Erdös while accepting an honorary degree — and that, some say, counts as a joint publication.