Directed path graphs

The concept of a line digraph is generalized to that of a directed path graph. The directed path graph $\overrightarrow P_k(D)$ of a digraph D is obtained by representing the directed paths on k vertices of D by vertices. Two vertices are joined by an arc whenever the corresponding directed paths in D form a directed path on k + 1 vertices or form a directed cycle on k vertices in D. Several properties of $\overrightarrow P_k(D)$ are studied, in particular with respect to isomorphism and traversability

How tough is toughness?

The concept of toughness was introduced by Chvátal [34] more than forty years ago. Toughness resembles vertex connectivity, but is different in the sense that it takes into account what the effect of deleting a vertex cut is on the number of resulting components. As we will see, this difference has major consequences in terms of computational complexity and on the implications with respect to cycle structure, in particular the existence of Hamilton cycles and k-factors

A note on a conjecture concerning tree-partitioning 3-regular graphs

If G is a 4-connected maximal planar graph, then G is hamiltonian (by a theorem\ud of Whitney), implying that its dual graph G� is a cyclically 4-edge connected 3-\ud regular planar graph admitting a partition of the vertex set into two parts, each\ud inducing a tree in G�, a so-called tree-partition. It is a natural question whether\ud each cyclically 4-edge connected 3-regular graph admits such a tree-partition.\ud This was conjectured by Jaeger, and recently independently by the �rst author.\ud The main result of this note shows that each connected 3-regular graph on n\ud vertices admits a partition of the vertex set into two sets such that precisely\ud 12\ud n+2 edges have end vertices in each set. This is a necessary condition for having\ud a tree-partition. We also show that not all cyclically 3-edge connected 3-regular\ud (planar) graphs admit a tree-partition, and present the smallest counterexample

On the complexity of dominating set problems related to the minimum all-ones problem

The minimum all-ones problem and the connected odd dominating set problem were shown to be NP-complete in different papers for general graphs, while they are solvable in linear time (or trivial) for trees, unicyclic graphs, and series-parallel graphs. The complexity of both problems when restricted to bipartite graphs was raised as an open question. Here we solve both problems. For this purpose, we introduce the related decision problem of the existence of an odd dominating set without isolated vertices, and study its complexity. Our main result shows that this new problem is NP-complete, even when restricted to bipartite graphs. We use this result to deduce that the minimum all-ones problem and the connected odd dominating set problem are also NP-complete for bipartite graphs. We show that all three problems are solvable in linear time for graphs with bounded treewidth. We also show that the new problem remains NP-complete when restricted to other graph classes, e.g., planar graphs, graphs with girth at least five, and graphs with a small maximum degree, in particular 3-regular graphs. \ud \u

Spanning trees with many or few colors in edge-colored graphs

Given a graph G = (V,E) and a (not necessarily proper) edge-coloring of G, we consider the complexity of finding a spanning tree of G with as many different colors as possible, and of finding one with as few different colors as possible. We show that the first problem is equivalent to finding a common independent set of maximum cardinality in two matroids, implying that there is a polynomial algorithm. We use the minimum dominating set problem to show that the second problem is NP-hard

Isomorphisms and traversability of directed path graphs

The concept of a line digraph is generalized to that of a directed path graph. The directed path graph Pk(D) of a digraph D is obtained by representing the directed paths on k vertices of D by vertices. Two vertices are joined by an arc whenever the corresponding directed paths in D form a directed path on k+1 vertices or form a directed cycle on k vertices in D. In this introductory paper several properties of P3(D) are studied, in particular with respect to isomorphism and traversability. In our main results, we characterize all digraphs D with P3(D) ≅ D, we show that P3(D1) ≅ P3(D2) "almost always'' implies D1 ≅ D2, and we characterize all digraphs with Eulerian or Hamiltonian P3-graphs

Decompositions of graphs based on a new graph product

Recently, we have introduced a new graph product, motivated by applications in the context of synchronising periodic real-time processes. This vertex-removing synchronised product (VRSP) is based on modifications of the well-known Cartesian product, and closely related to the synchronised product due to Wöhrle and Thomas. Here, we recall the definition of the VRSP and use it to define two different decompositions of graphs. Although our main results apply to directed labelled acyclic multigraphs, the VRSP can also be used to decompose any undirected graph of order at least 4 into two smaller graphs

A σ3 type condition for heavy cycles in weighted graphs

A weighted graph is a graph in which each edge e is assigned a non-negative number w(e), called the weight of e. The weight of a cycle is the sum of the weights of its edges. The weighted degree dw(v) of a vertex v is the sum of the weights of the edges incident with v. In this paper, we prove the following result: Suppose G is a 2-connected weighted graph which satisfies the following conditions: 1. The weighted degree sum of any three independent vertices is at least m; 2. w(xz)=w(yz) for every vertex z∈N(x)∩ N(y) with d(x,y)=2; 3. In every triangle T of G, either all edges of T have different weights or all edges of T have the same weight. Then G contains either a Hamilton cycle or a cycle of weight at least 2m/3. This generalizes a theorem of Fournier and Fraisse on the existence of long cycles in k-connected unweighted graphs in the case k=2. Our proof of the above result also suggests a new proof to the theorem of Fournier and Fraisse in the case k=2

Three results on cycle-wheel Ramsey numbers

Given two graphs G1 and G2, the Ramsey number R(G1,G2) is the smallest integer N such that, for any graph G of order N, either G1 is a subgraph of G, or G2 is a subgraph of the complement of G. We consider the case that G1 is a cycle and G2 is a (generalized) wheel. We expand the knowledge on exact values of Ramsey numbers in three directions: large cycles versus wheels of odd order; large wheels versus cycles of even order; and large cycles versus generalized odd wheels