String factorisations with maximum or minimum dimension.

In this paper we consider two problems concerning string factorisation. Specifically given a string w and an integer k find a factorisation of w where each factor has length bounded by k and has the minimum (the F-Min-D problem) or the maximum (the F-Max-D problem) number of different factors. The F-Min-D has been proved to be NP-hard even if k = 2 in [9] and for this case we provide a 3/2-approximation algorithm. The F-Max-D problem, up to our knowledge has not been considered in the literature. We show that this problem is NP-hard for any k ≥ 3. In view of this we propose a 2-approximation algorithm (for any k) and an FPT algorithm w.r.t. parameter max ⁡ { k , | Σ | }. [ABSTRACT FROM AUTHOR]