Geodesic
Minimum vertex ranking spanning tree problem for chordal and proper interval graphs
Discussiones Mathematicae. Graph Theory, Tome 29 (2009) no. 2, pp. 253-261

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A vertex k-ranking of a simple graph is a coloring of its vertices 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 spanning 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 vertex 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.
Keywords: computational complexity, vertex ranking, spanning tree 
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     title = {Minimum vertex ranking spanning tree problem for chordal and proper interval graphs},
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Dereniowski, Dariusz. Minimum vertex ranking spanning tree problem for chordal and proper interval graphs. Discussiones Mathematicae. Graph Theory, Tome 29 (2009) no. 2, pp. 253-261. http://geodesic.mathdoc.fr/item/DMGT_2009_29_2_a3/