Abstract
The class of Galled-Tree Explainable (GaTEx) graphs has just recently been discovered as a natural generalization of cographs. Cographs are precisely those graphs that can be uniquely represented by a rooted tree where the leaves of the tree correspond to the vertices of the graph. As a generalization, GaTEx graphs are precisely those graphs that can be uniquely represented by a particular rooted directed acyclic graph (called galled-tree).
We consider here four prominent problems that are, in general, NP-hard: computing the size \(\omega (G)\) of a maximum clique, the size \(\chi (G)\) of an optimal vertex-coloring and the size \(\alpha (G)\) of a maximum independent set of a given graph G as well as determining whether a graph is perfectly orderable. We show here that \(\omega (G)\), \(\chi (G)\), \(\alpha (G)\) can be computed in linear-time for GaTEx graphs G. The crucial idea for the linear-time algorithms is to avoid working on the GaTEx graphs G directly, but to use the galled-trees that explain G as a guide for the algorithms to compute these invariants. In particular, we show first how to employ the galled-tree structure to compute a perfect ordering of GaTEx graphs in linear-time which is then used to determine \(\omega (G)\), \(\chi (G)\), \(\alpha (G)\).
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Hellmuth, M., Scholz, G.E. (2024). Linear Time Algorithms for NP-Hard Problems Restricted to GaTEx Graphs. In: Wu, W., Tong, G. (eds) Computing and Combinatorics. COCOON 2023. Lecture Notes in Computer Science, vol 14422. Springer, Cham. https://doi.org/10.1007/978-3-031-49190-0_8
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