82 research outputs found
Longest Path and Cycle Transversal and Gallai Families
A longest path transversal in a graph G is a set of vertices S of G such that every longest path in G has a vertex in S. The longest path transversal number of a graph G is the size of a smallest longest path transversal in G and is denoted lpt(G). Similarly, a longest cycle transversal is a set of vertices S in a graph G such that every longest cycle in G has a vertex in S. The longest cycle transversal number of a graph G is the size of a smallest longest cycle transversal in G and is denoted lct(G). A Gallai family is a family of graphs whose connected members have longest path transversal number 1. In this paper we find several Gallai families and give upper bounds on lpt(G) and lct(G) for general graphs and chordal graphs in terms of |V(G)|
On P 4 -transversals of Chordal Graphs
Abstract A P 4 -transversal of a graph G is a set of vertices T which meets every P 4 of G. A P 4 -transversal T is called stable if there are no edges in the subgraph of G induced by T . It has been previously shown by Hoàng and Le that it is N P -complete to decide whether a comparability (and hence perfect) graph G has a stable P 4 -transversal. In the following we show that the problem is N P -complete for chordal graphs. We apply this result to show that two related problems of deciding whether a chordal graph has a P 3 -free P 4 -transversal, and deciding whether a chordal graph has a P 4 -free P 4 -transversal (also known as a two-sided P 4 -transversal) are both N Pcomplete. Additionally, we strengthen the main results to strongly chordal graphs
Decycling a graph by the removal of a matching: new algorithmic and structural aspects in some classes of graphs
A graph is {\em matching-decyclable} if it has a matching such that
is acyclic. Deciding whether is matching-decyclable is an NP-complete
problem even if is 2-connected, planar, and subcubic. In this work we
present results on matching-decyclability in the following classes: Hamiltonian
subcubic graphs, chordal graphs, and distance-hereditary graphs. In Hamiltonian
subcubic graphs we show that deciding matching-decyclability is NP-complete
even if there are exactly two vertices of degree two. For chordal and
distance-hereditary graphs, we present characterizations of
matching-decyclability that lead to -time recognition algorithms
Neighborhood Inclusions for Minimal Dominating Sets Enumeration: Linear and Polynomial Delay Algorithms in P_7 - Free and P_8 - Free Chordal Graphs
In [M. M. Kant\'e, V. Limouzy, A. Mary, and L. Nourine. On the enumeration of
minimal dominating sets and related notions. SIAM Journal on Discrete
Mathematics, 28(4):1916-1929, 2014] the authors give an delay
algorithm based on neighborhood inclusions for the enumeration of minimal
dominating sets in split and -free chordal graphs. In this paper, we
investigate generalizations of this technique to -free chordal graphs for
larger integers . In particular, we give and delays
algorithms in the classes of -free and -free chordal graphs. As for
-free chordal graphs for , we give evidence that such a technique
is inefficient as a key step of the algorithm, namely the irredundant extension
problem, becomes NP-complete.Comment: 16 pages, 3 figure
Efficient enumeration of maximal split subgraphs and sub-cographs and related classes
In this paper, we are interested in algorithms that take in input an
arbitrary graph , and that enumerate in output all the (inclusion-wise)
maximal "subgraphs" of which fulfil a given property . All over this
paper, we study several different properties , and the notion of subgraph
under consideration (induced or not) will vary from a result to another.
More precisely, we present efficient algorithms to list all maximal split
subgraphs, sub-cographs and some subclasses of cographs of a given input graph.
All the algorithms presented here run in polynomial delay, and moreover for
split graphs it only requires polynomial space. In order to develop an
algorithm for maximal split (edge-)subgraphs, we establish a bijection between
the maximal split subgraphs and the maximal independent sets of an auxiliary
graph. For cographs and some subclasses , the algorithms rely on a framework
recently introduced by Conte & Uno called Proximity Search. Finally we consider
the extension problem, which consists in deciding if there exists a maximal
induced subgraph satisfying a property that contains a set of prescribed
vertices and that avoids another set of vertices. We show that this problem is
NP-complete for every "interesting" hereditary property . We extend the
hardness result to some specific edge version of the extension problem
Three problems on well-partitioned chordal graphs
In this work, we solve three problems on well-partitioned chordal graphs. First, we show that every connected (resp., 2-connected) well-partitioned chordal graph has a vertex that intersects all longest paths (resp., longest cycles). It is an open problem [Balister et al., Comb. Probab. Comput. 2004] whether the same holds for chordal graphs. Similarly, we show that every connected well-partitioned chordal graph admits a (polynomial-time constructible) tree 3-spanner, while the complexity status of the Tree 3-Spanner problem remains open on chordal graphs [Brandstädt et al., Theor. Comput. Sci. 2004]. Finally, we show that the problem of finding a minimum-size geodetic set is polynomial-time solvable on well-partitioned chordal graphs. This is the first example of a problem that is NP -hard on chordal graphs and polynomial-time solvable on well-partitioned chordal graphs. Altogether, these results reinforce the significance of this recently defined graph class as a tool to tackle problems that are hard or unsolved on chordal graphs.acceptedVersio
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