Recognizing vertex intersection graphs of paths on bounded degree trees

An (h,s,t)-representation of a graph G consists of a collection of subtrees of a tree T, where each subtree corresponds to a vertex of G such that (i) the maximum degree of T is at most h, (ii) every subtree has maximum degree at most s, (iii) there is an edge between two vertices in the graph G if and only if the corresponding subtrees have at least t vertices in common in T. The class of graphs that has an (h,s,t)-representation is denoted by [h,s,t]. An undirected graph G is called a VPT graph if it is the vertex intersection graph of a family of paths in a tree. Thus, [h,2,1] graphs are the VPT graphs that can be represented in a tree with maximum degree at most h. In this paper we characterize [h,2,1] graphs using chromatic number. We show that the problem of deciding whether a given VPT graph belongs to [h,2,1] is NP-complete, while the problem of deciding whether the graph belongs to [h,2,1]-[h-1,2,1] is NP-hard. Both problems remain hard even when restricted to VPT∩Split. Additionally, we present a non-trivial subclass of VPT∩Split in which these problems are polynomial time solvable.Facultad de Ciencias Exacta

Characterizing paths graphs on bounded degree trees by minimal forbidden induced subgraphs

An undirected graph G is called a VPT graph if it is the vertex intersection graph of a family of paths in a tree. The class of graphs which admit a VPT representation in a host tree with maximum degree at most h is denoted by [h,2,1]. The classes [h,2,1] are closed under taking induced subgraphs, therefore each one can be characterized by a family of minimal forbidden induced subgraphs. In this paper we associate the minimal forbidden induced subgraphs for [h,2,1] which are VPT with (color) h-critical graphs. We describe how to obtain minimal forbidden induced subgraphs from critical graphs, even more, we show that the family of graphs obtained using our procedure is exactly the family of VPT minimal forbidden induced subgraphs for [h,2,1]. The members of this family together with the minimal forbidden induced subgraphs for VPT (Lévêque et al., 2009; Tondato, 2009), are the minimal forbidden induced subgraphs for [h,2,1], with h ≥ 3. By taking h=3 we obtain a characterization by minimal forbidden induced subgraphs of the class V PT∩EPT=EPT∩Chordal=[3,2,2]=[3,2,1] (see Golumbic and Jamison, 1985).Facultad de Ciencias Exacta

Recognizing vertex intersection graphs of paths on bounded degree trees

An (h,s,t)-representation of a graph G consists of a collection of subtrees of a tree T, where each subtree corresponds to a vertex of G such that (i) the maximum degree of T is at most h, (ii) every subtree has maximum degree at most s, (iii) there is an edge between two vertices in the graph G if and only if the corresponding subtrees have at least t vertices in common in T. The class of graphs that has an (h,s,t)-representation is denoted by [h,s,t]. An undirected graph G is called a VPT graph if it is the vertex intersection graph of a family of paths in a tree. Thus, [h,2,1] graphs are the VPT graphs that can be represented in a tree with maximum degree at most h. In this paper we characterize [h,2,1] graphs using chromatic number. We show that the problem of deciding whether a given VPT graph belongs to [h,2,1] is NP-complete, while the problem of deciding whether the graph belongs to [h,2,1]-[h-1,2,1] is NP-hard. Both problems remain hard even when restricted to VPT∩Split. Additionally, we present a non-trivial subclass of VPT∩Split in which these problems are polynomial time solvable.Facultad de Ciencias Exacta

Constant tolerance intersection graphs of subtrees of a tree

AbstractA chordal graph is the intersection graph of a family of subtrees of a host tree. In this paper we generalize this. A graph G=(V,E) has an (h,s,t)-representation if there exists a host tree T of maximum degree at most h, and a family of subtrees {Sv}v∈V of T, all of maximum degree at most s, such that uv∈E if and only if |Su∩Sv|⩾t. For given h,s, and t, there exist infinitely many forbidden induced subgraphs for the class of (h,s,t)-graphs. On the other hand, for fixed h⩾s⩾3, every graph is an (h,s,t)-graph provided that we take t large enough. Under certain conditions representations of larger graphs can be obtained from those of smaller ones by amalgamation procedures. Other representability and non-representability results are presented as well

Characterizing paths graphs on bounded degree trees by minimal forbidden induced subgraphs

An undirected graph G is called a VPT graph if it is the vertex intersection graph of a family of paths in a tree. The class of graphs which admit a VPT representation in a host tree with maximum degree at most h is denoted by [h,2,1]. The classes [h,2,1] are closed under taking induced subgraphs, therefore each one can be characterized by a family of minimal forbidden induced subgraphs. In this paper we associate the minimal forbidden induced subgraphs for [h,2,1] which are VPT with (color) h-critical graphs. We describe how to obtain minimal forbidden induced subgraphs from critical graphs, even more, we show that the family of graphs obtained using our procedure is exactly the family of VPT minimal forbidden induced subgraphs for [h,2,1]. The members of this family together with the minimal forbidden induced subgraphs for VPT (Lévêque et al., 2009; Tondato, 2009), are the minimal forbidden induced subgraphs for [h,2,1], with h ≥ 3. By taking h=3 we obtain a characterization by minimal forbidden induced subgraphs of the class V PT∩EPT=EPT∩Chordal=[3,2,2]=[3,2,1] (see Golumbic and Jamison, 1985).Facultad de Ciencias Exacta