See the on-line encyclopedia of integer sequences:
RATS: Reverse Add Then Sort the digits applied to previous term, starting with 1.
It is conjectured that no matter what the starting term is, repeatedly applying RATS leads either to this sequence or into a cycle of finite length, such as those in A066710 and A066711.
Let a(n) = k, form m by Reversing the digits of k, Add m to k Then Sort the digits of the sum into increasing order to get a(n+1).
668 -> 668 + 866 = 1534 -> 1345.