Maximum Letter-Duplicated Subsequence (short paper)

In this contribution we consider Max-LL-DUP, an optimization problem that asks for a letter-duplicated subsequence of an input string that contains the maximum number of letters of the alphabet over which the input string is defined. When each letter has at most four occurrences in the input string, we prove that the problem is APX-hard. Then, we give a linear-time algorithm when each letter has at most three occurrences in the input string.