Paul ErdÅ ` s , also PÃ l ErdÅ ` s , in English Paul Erdos or Paul ErdÃ s ( March 26 , 1913 â `` September 20 , 1996 ) , was a famous Hungarian-born mathematician . Paul ErdÅ ` s ( occasionally spelled Erdos or ErdÃ s ; , ; 26 March 1913 â `` 20 September 1996 ) was a Hungarian mathematician .