Original: Also, Michael Barr suggests "rational ErdÅ 's numbers", generalizing the idea that a person who has written p joint papers with ErdÅ' s should be assigned ErdÅ 's number 1\/p.
Simplified: Anyone who directly worked with Paul ErdÅ 's has an ErdÅ' s number of 1. Anyone who works with those collaborators who have a ErdÅ 's number of 1, have an ErdÅ' s number of 2.