131221
De uitleg heb ik alleen in het Engels kunnen vinden:
Again, the system behind the sequence is that each number (except the first one of the sequence) "describes" the previous number. Now, however, the number of occurrences of each digit is counted. So 1231 means one "2" and three times a "1", and 131221 means one "3", one "2", and two times a "1".