Abstract
A vertex k-ranking of a simple graph is a coloring of its ver- tices with k colors in such a way that each path connecting two vertices of the same color contains a vertex with a bigger color. Consider the minimum vertex ranking spanning tree (MVRST) problem where the goal is to find a spanning tree of a given graph G which has a vertex ranking using the minimal number of colors over vertex rankings of all spanning trees of G. K. Miyata et. al. proved in (Np-hardness proof and an approximation algorithm for the minimum vertex ranking span- ning tree problem, Discrete Appl. Math. 154 (2006) 2402-2410) that the decision problem: given a simple graph G, decide whether there exists a spanning tree T of G such that T has a vertex 4-ranking, is NP- complete. In this paper we improve this result by proving NP-hardness of finding for a given chordal graph its spanning tree having v ertex 3- ranking. This bound is the best possible. On the other hand we prove that MVRST problem can be solved in linear time for proper interval graphs.
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