Because he wrote so many articles , friends created ErdÅ ` s number . ErdÅ ` s has a number of 0 ( for being himself ) , and his direct collaborators were given the number 1 . Because of his prolific output , friends created the ErdÅ ` s number as a humorous tribute ; ErdÅ ` s alone was assigned the ErdÅ ` s number of 0 ( for being himself ) , while his immediate collaborators could claim an ErdÅ ` s number of 1 , their collaborators have ErdÅ ` s number at most 2 , and so on .