A sequence produced by the instructions "reverse, add to the original, then sort the digits." For example, after 668, the next iteration is given by
 |
so the next term is 1345.
Applied to 1, the sequence gives 1, 2, 4, 8, 16, 77, 145, 668, 1345, 6677, 13444, 55778, 133345, 666677, 1333444, 5567777, 12333445, 66666677, 133333444, 556667777, 1233334444, 5566667777, 12333334444, 55666667777, 123333334444, 556666667777, 1233333334444, ... (OEIS A004000).
Conway conjectured that an initial number leads to a divergent period-two pattern (such as the above in which the numbers of threes and sixes in the middles of alternate terms steadily increase) or to a cycle (Guy 2004, p. 404).
The lengths of the cycles obtained by starting with 
, 2, ... are 0, 0, 8, 0, 0, 8, 0, 0, 2, 0, ... (OEIS A114611),
where a 0 indicates that the sequence diverges.
The following table summarizes the first few values of 
leading to a period of length 
. There are no other periods of length 50 or less for 
(E. W. Weisstein,
Dec. 19, 2005).
 | OEIS |  with period  |
 | A001651 | 1, 2, 4, 5, 7, 8, 10, 11, 13, 14, 16, 17, 19, 20, ... |
| 2 | A114612 | 9, 18, 27, 36, 45, 54, 63, 69, 72, 78, 81, 87, 90, 96, ... |
| 3 | A114613 | 20169, 20709, 21159, 22149, 23139, 24129, 25119, 26109, ... |
| 8 | A114614 | 3, 6, 12, 15, 21, 24, 30, 33, 39, 42, 48, 51, 57, 60, 66, ... |
| 14 | A114615 | 6999, 7089, 7179, 7269, 7359, 7449, 7539, 7629, ... |
| 18 | A114616 | 29, 38, 47, 49, 56, 58, 65, 67, 74, 76, 83, 85, 92, 94, ... |
