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A New Graph Parameter to Measure Linearity

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Abstract

Since its introduction to recognize chordal graphs by Rose, Tarjan, and Lueker, Lexicographic Breadth First Search (LexBFS) has been used to come up with simple, often linear time, algorithms on various classes of graphs. These algorithms are usually multi-sweep algorithms; that is they compute LexBFS orderings σ1,…,σkσ1,…,σk , where σiσi is used to break ties for σi+1σi+1 . Since the number of LexBFS orderings for a graph is finite, this infinite sequence {σi}{σi} must have a loop, i.e. a multi-sweep algorithm will loop back to compute σjσj , for some j. We study this new graph invariant, LexCycle(G), defined as the maximum length of a cycle of vertex orderings obtained via a sequence of LexBFS ++ . In this work, we focus on graph classes with small LexCycle. We give evidence that a small LexCycle often leads to linear structure that has been exploited algorithmically on a number of graph classes. In particular, we show that for proper interval, interval, co-bipartite, domino-free cocomparability graphs, as well as trees, there exists two orderings σσ and ττ such that σ=LexBFS+(τ)σ=LexBFS+(τ) and τ=LexBFS+(σ)τ=LexBFS+(σ) . One of the consequences of these results is the simplest algorithm to compute a transitive orientation for these graph classes. It was conjectured by Stacho [2015] that LexCycle is at most the asteroidal number of the graph class, we disprove this conjecture by giving a construction for which LexCycle(G)>an(G)LexCycle(G)>an(G) , the asteroidal number of G.

Dates and versions

hal-01672521 , version 1 (26-12-2017)

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Pierre Charbit, Michel Habib, Lalla Mouatadid, Reza Naserasr. A New Graph Parameter to Measure Linearity. COCOA 2017 - 11th Annual International Conference on Combinatorial Optimization and Applications, Dec 2017, Shanghai, China. pp.154-168, ⟨10.1007/978-3-319-71147-8_11⟩. ⟨hal-01672521⟩
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