Results For Planar Graphs Research Articles

A Stallings type theorem for quasi-transitive graphs

We consider locally finite, connected, quasi-transitive graphs and show that every such graph with more than one end is a tree amalgamation of two other such graphs. This can be seen as a graph-theoretical version of Stallings' splitting theorem for multi-ended finitely generated groups and indeed it implies this theorem. Our result also leads to a characterisation of accessible graphs. We obtain applications of our results for planar graphs (answering a variant of a question by Mohar in the affirmative) and graphs without thick ends.

$$L(p,q)$$ L ( p , q ) -labeling and integer tension of a graph embedded on torus

The $$L(p, q)$$L(p,q)-labeling arises from the optimization problem of channel assignment in communication networks. For two non-negative integers $$p$$p and $$q$$q, an $$L(p,q)$$L(p,q)-labeling $$c$$c of a graph $$G$$G is an assignment of non-negative integers to the vertices of $$G$$G such that adjacent vertices are labelled using colors at least $$p$$p apart, and vertices with distance two are labelled using colors at least $$q$$q apart. In this paper we establish a connection between an $$L(p, q)$$L(p,q)-labeling and an integer tension of a graph, which extends a corresponding result for planar graphs. This connection provides us with an effective way to design an $$L(p, q)$$L(p,q)-labeling for non-planar graphs, in particular for graphs embedded on torus, by choosing a proper cycle basis consisting of facial cycles and some specified cycles of the embedded graph. As an application, we use this method to optimize the edge span for the Cartesian product of two cycles.

The Complexity of the Empire Colouring Problem

We investigate the computational complexity of the empire colouring problem (as defined by Percy Heawood in 1890) for maps containing empires formed by exactly $r > 1$ countries each. We prove that the problem can be solved in polynomial time using $s$ colours on maps whose underlying adjacency graph has no induced subgraph of average degree larger than $s/r$. However, if $s \geq 3$, the problem is NP-hard even if the graph is a forest of paths of arbitrary lengths (for any $r \geq 2$, provided $s < 2r - \sqrt(2r + 1/4+ 3/2)$. Furthermore we obtain a complete characterization of the problem's complexity for the case when the input graph is a tree, whereas our result for arbitrary planar graphs fall just short of a similar dichotomy. Specifically, we prove that the empire colouring problem is NP-hard for trees, for any $r \geq 2$, if $3 \leq s \leq 2r-1$ (and polynomial time solvable otherwise). For arbitrary planar graphs we prove NP-hardness if $s<7$ for $r=2$, and $s < 6r-3$, for $r \geq 3$. The result for planar graphs also proves the NP-hardness of colouring with less than 7 colours graphs of thickness two and less than $6r-3$ colours graphs of thickness $r \geq 3$.

Topological directions in Cops and Robbers

We survey results at the intersection of topological graph theory and the game of Cops and Robbers, focusing on results, conjectures, and open problems for the cop number of a graph embedded on a surface. After a discussion on results for planar graphs, we consider graphs of higher genus. In 2001, Schroeder conjectured that if a graph has genus $g,$ then its cop number is at most $g + 3.$ While Schroeder's bound is known to hold for planar and toroidal graphs, the case for graphs with higher genus remains open. We consider the capture time of graphs on surfaces and examine results for embeddings of graphs on non-orientable surfaces. We present a conjecture by the second author, and in addition, we survey results for the lazy cop number, directed graphs, and Zombies and Survivors.