via V. Faber. (Paul Erdös has Erdö s number 0, his co-authors have Erdös number 1, the other co-authors of his co-authors have Erdös number 2, and so on. The Erdös number has been introduced by C. Goffman in his paper "And what is your Erdös number?" Amer. Math. Monthly 76(1969), 791.)