88 research outputs found
Characterizations and algorithms for generalized Cops and Robbers games
We propose a definition of generalized Cops and Robbers games where there are
two players, the Pursuer and the Evader, who each move via prescribed rules. If
the Pursuer can ensure that the game enters into a fixed set of final
positions, then the Pursuer wins; otherwise, the Evader wins. A relational
characterization of the games where the Pursuer wins is provided. A precise
formula is given for the length of the game, along with an algorithm for
computing if the Pursuer has a winning strategy whose complexity is a function
of the parameters of the game. For games where the position of one player does
not affect the available moves of he other, a vertex elimination ordering
characterization, analogous to a cop-win ordering, is given for when the
Pursuer has a winning strategy
A Study of -dipath Colourings of Oriented Graphs
We examine -colourings of oriented graphs in which, for a fixed integer , vertices joined by a directed path of length at most must be
assigned different colours. A homomorphism model that extends the ideas of
Sherk for the case is described. Dichotomy theorems for the complexity of
the problem of deciding, for fixed and , whether there exists such a
-colouring are proved.Comment: 14 page
Graph homomorphisms with infinite targets
AbstractLet H be a fixed graph whose vertices are called colours. Informally, an H-colouring of a graph G is an assignment of these colours to the vertices of G such that adjacent vertices receive adjacent colours. We introduce a new tool for proving NP-completeness of H-colouring problems, which unifies all methods used previously. As an application we extend, to infinite graphs of bounded degree, the theorem of Hell and Nešetřil that classifies finite H-colouring problems by complexity
Building blocks for the variety of absolute retracts
AbstractGiven a graph H with a labelled subgraph G, a retraction of H to G is a homomorphism r:H→G such that r(x)=x for all vertices x in G. We call G a retract of H. While deciding the existence of a retraction to a fixed graph G is NP-complete in general, necessary and sufficient conditions have been provided for certain classes of graphs in terms of holes, see for example Hell and Rival.For any integer k⩾2 we describe a collection of graphs that generate the variety ARk of graphs G with the property that G is a retract of H whenever G is a subgraph of H and no hole in G of size at most k is filled by a vertex of H. We also prove that ARk⊂NUFk+1, where NUFk+1 is the variety of graphs that admit a near unanimity function of arity k+1
Characterizations of k-copwin graphs
AbstractWe give two characterizations of the graphs on which k cops have a winning strategy in the game of Cops and Robber. One of these is in terms of an order relation, and one is in terms of a vertex ordering. Both generalize characterizations known for the case k=1
Oriented Colourings of Graphs with Maximum Degree Three and Four
We show that any orientation of a graph with maximum degree three has an
oriented 9-colouring, and that any orientation of a graph with maximum degree
four has an oriented 69-colouring. These results improve the best known upper
bounds of 11 and 80, respectively
2-limited broadcast domination in grid graphs
We establish upper and lower bounds for the 2-limited broadcast domination
number of various grid graphs, in particular the Cartesian product of two
paths, a path and a cycle, and two cycles. The upper bounds are derived by
explicit constructions. The lower bounds are obtained via linear programming
duality by finding lower bounds for the fractional 2-limited multipacking
numbers of these graphs
Colourings of -coloured mixed graphs
A mixed graph is, informally, an object obtained from a simple undirected
graph by choosing an orientation for a subset of its edges. A mixed graph is
-coloured if each edge is assigned one of colours, and each
arc is assigned one of colours. Oriented graphs are -coloured mixed graphs, and 2-edge-coloured graphs are -coloured
mixed graphs. We show that results of Sopena for vertex colourings of oriented
graphs, and of Kostochka, Sopena and Zhu for vertex colourings oriented graphs
and 2-edge-coloured graphs, are special cases of results about vertex
colourings of -coloured mixed graphs. Both of these can be regarded as
a version of Brooks' Theorem.Comment: 7 pages, no figure
Homomorphically Full Oriented Graphs
Homomorphically full graphs are those for which every homomorphic image is
isomorphic to a subgraph. We extend the definition of homomorphically full to
oriented graphs in two different ways. For the first of these, we show that
homomorphically full oriented graphs arise as quasi-transitive orientations of
homomorphically full graphs. This in turn yields an efficient recognition and
construction algorithms for these homomorphically full oriented graphs. For the
second one, we show that the related recognition problem is GI-hard, and that
the problem of deciding if a graph admits a homomorphically full orientation is
NP-complete. In doing so we show the problem of deciding if two given oriented
cliques are isomorphic is GI-complete
Bounds for the -Eternal Domination Number of a Graph
Mobile guards on the vertices of a graph are used to defend the graph against an infinite sequence of attacks on vertices. A guard must move from a neighboring vertex to an attacked vertex (we assume attacks happen only at vertices containing no guard and that each vertex contains at most one guard). More than one guard is allowed to move in response to an attack. The -eternaldomination number, \edom(G), of a graph is the minimum number of guards needed to defend against any such sequence. We show that if is a connected graph with minimum degree at least~ and of order~, then \edom(G) \le \left\lfloor \frac{n-1}{2} \right\rfloor, and this bound is tight. We also prove that if is a cubic bipartite graph of order~, then \edom(G) \le \frac{7n}{16}
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