Irregularity of expansions and Pell graphs

For a graph $G$ the imbalance of an edge $uv$ of $G$ is $|deg_G(u)-deg_G(v)|$. Irregularity of a graph $G$ is defined as the sum of imbalances over all edges of $G$. In this paper we consider expansions and Pell graphs. If $H$ is an expansion of $G$ with respect to the sets $V_1$ and $V_2$, we express the irregularity of $H$ using $G, V_1$ and $V_2$. For Pell graphs the imbalance of their edges is studied. The number of edges of a Pell graph with a fixed imbalance $k$ is expressed. Using these results the irregularity and the $\sigma$-index of Pell graphs are given

Span of a Graph: Keeping the Safety Distance

Inspired by Lelek's idea from \cite{Lelek}, we introduce the novel notion of the span of graphs. Using this, we solve the problem of determining the \emph{maximal safety distance} two players can keep at all times while traversing a graph. Moreover, their moves must be made with respect to certain move rules. For this purpose, we introduce different variants of a span of a given connected graph. All the variants model the maximum safety distance kept by two players in a graph traversal, where the players may only move with accordance to a specific set of rules, and their goal: visit either all vertices, or all edges. For each variant, we show that the solution can be obtained by considering only connected subgraphs of a graph product and the projections to the factors. We characterise graphs in which it is impossible to keep a positive safety distance at all moments in time. Finally, we present a polynomial time algorithm that determines the chosen span variant of a given graph

A note on the Thue chromatic number of lexicographic products of graphs

A sequence is called non-repetitive if none of its subsequences forms a repetition (a sequence r1r2 · · · r2n such that ri = rn+i for all 1 ≤ i ≤ n). Let G be a graph whose vertices are coloured. A colouring ϕ of the graph G is non-repetitive if the sequence of colours on every path in G is non-repetitive. The Thue chromatic number, denoted by π(G), is the minimum number of colours of a non-repetitive colouring of G

Maximum cardinality resonant sets and maximal alternating sets of hexagonal systems

AbstractIt is shown that the Clar number can be arbitrarily larger than the cardinality of a maximal alternating set. In particular, a maximal alternating set of a hexagonal system need not contain a maximum cardinality resonant set, thus disproving a previously stated conjecture. It is known that maximum cardinality resonant sets and maximal alternating sets are canonical, but the proofs of these two theorems are analogous and lengthy. A new conjecture is proposed and it is shown that the validity of the conjecture allows short proofs of the aforementioned two results. The conjecture holds for catacondensed hexagonal systems and for all normal hexagonal systems up to ten hexagons. Also, it is shown that the Fries number can be arbitrarily larger than the Clar number

A new characterization and a recognition algorithm of Lucas cubes

Fibonacci and Lucas cubes are induced subgraphs of hypercubes obtained by excluding certain binary strings from the vertex set. They appear as models for interconnection networks, as well as in chemistry. We derive a characterization of Lucas cubes that is based on a peripheral expansion of a unique convex subgraph of an appropriate Fibonacci cube.This serves as the foundation for a recognition algorithm of Lucas cubes that runs in linear time

1-factors and characterization of reducible faces of plane elementary bipartite graphs

As a general case of molecular graphs of benzenoid hydrocarbons, we study plane bipartite graphs with Kekulé structures (1-factors). A bipartite graph ▫$G$▫ is called elementary if ▫$G$▫ is connected and every edge belongs to a 1-factor of ▫$G$▫. Some properties of the minimal and the maximal 1-factor of a plane elementary graph are given. A peripheral face ▫$f$▫ of a plane elementary graph is reducible, if the removal of the internal vertices and edges of the path that is the intersection of ▫$f$▫ and the outer cycle of ▫$G$▫ results in an elementary graph. We characterize the reducible faces of a plane elementary bipartite graph. This result generalizes the characterization of reducible faces of an elementary benzenoid graph