[math-fun] Add the identical digits in a substring

Hello Math-Fun, I'm looking for loops with the hereunder rules: -start with a(1) -if a substring of a(1) is made of identical digits, replace the said substring with the sum of its digits -iterate until there are no more substrings with identical digits in a(1) -double a(1) and start the pro- cedure again. Ex. a(1)= 55027771 a(1)=1002211 a(1)=1042 a(2)=2084 a(3)=4168 a(4)=8336 a(4)=866 a(4)=812 a(5)=1624 etc. 18 enters in the loop 18-36-72-144-(18) Best, É.