On the hardness of inferring phylogenies from triplet-dissimilarities

This work considers the problem of reconstructing a phylogenetic tree from triplet dissimilarities, which are dissimilarities defined over taxontriplets. Triplet dissimilarities are possibly the simplest generalization of pairwise dissimilarities, and were used for phylogenetic reconstructions in the past few years. We study the hardness of finding a tree best fitting a given triplet-dissimilarity table under the `∞ norm. We show that the corresponding decision problem is NP-hard and that the corresponding optimization problem cannot be approximated in polynomial time within a constant multiplicative factor smaller than 1.4. On the positive side, we present a polynomial time constant-rate approximation algorithm for this problem. We also address the issue of best-fit under maximal distortion, which corresponds to the largest ratio between matching entries in two triplet-dissimilarity tables. We show that it is NP-hard to approximate the corresponding optimization problem within any constant multiplicative factor.