Convex circuit free coloration of an oriented graph

We introduce the \textit{convex circuit-free coloration} and \textit{convex circuit-free chromatic number} $\overrightarrow{\chi_a}(\overrightarrow{G})$ of an oriented graph $\overrightarrow{G}$ and establish various basic results. We show that the problem of deciding if an oriented graph verifies $\chi_a( \overrightarrow{G}) \leq k$ is NP-complete if $k \geq 3$ and polynomial if $k \leq 2$. We exhibit an algorithm which finds the optimal convex circuit-free coloration for tournaments, and characterize the tournaments that are \textit{vertex-critical} for the convex circuit-free coloration

Lattice Points in Orthotopes and a Huge Polynomial Tutte Invariant of Weighted Gain Graphs

A gain graph is a graph whose edges are orientably labelled from a group. A weighted gain graph is a gain graph with vertex weights from an abelian semigroup, where the gain group is lattice ordered and acts on the weight semigroup. For weighted gain graphs we establish basic properties and we present general dichromatic and forest-expansion polynomials that are Tutte invariants (they satisfy Tutte's deletion-contraction and multiplicative identities). Our dichromatic polynomial includes the classical graph one by Tutte, Zaslavsky's two for gain graphs, Noble and Welsh's for graphs with positive integer weights, and that of rooted integral gain graphs by Forge and Zaslavsky. It is not a universal Tutte invariant of weighted gain graphs; that remains to be found. An evaluation of one example of our polynomial counts proper list colorations of the gain graph from a color set with a gain-group action. When the gain group is Z^d, the lists are order ideals in the integer lattice Z^d, and there are specified upper bounds on the colors, then there is a formula for the number of bounded proper colorations that is a piecewise polynomial function of the upper bounds, of degree nd where n is the order of the graph. This example leads to graph-theoretical formulas for the number of integer lattice points in an orthotope but outside a finite number of affinographic hyperplanes, and for the number of n x d integral matrices that lie between two specified matrices but not in any of certain subspaces defined by simple row equations.Comment: 32 pp. Submitted in 2007, extensive revisions in 2013 (!). V3: Added references, clarified examples. 35 p

An extensive English language bibliography on graph theory and its applications, supplement 1

Graph theory and its applications - bibliography, supplement

Inversion dans les tournois

We consider the transformation reversing all arcs of a subset $X$ of the vertex set of a tournament $T$. The \emph{index} of $T$, denoted by $i(T)$, is the smallest number of subsets that must be reversed to make $T$ acyclic. It turns out that critical tournaments and $(-1)$-critical tournaments can be defined in terms of inversions (at most two for the former, at most four for the latter). We interpret $i(T)$ as the minimum distance of $T$ to the transitive tournaments on the same vertex set, and we interpret the distance between two tournaments $T$ and $T'$ as the \emph{Boolean dimension} of a graph, namely the Boolean sum of $T$ and $T'$. On $n$ vertices, the maximum distance is at most $n-1$, whereas $i(n)$, the maximum of $i(T)$ over the tournaments on $n$ vertices, satisfies $\frac {n-1}{2} - \log_{2}n \leq i(n) \leq n-3$, for $n \geq 4$. Let $\mathcal{I}_{m}^{< \omega}$ (resp. $\mathcal{I}_{m}^{\leq \omega}$) be the class of finite (resp. at most countable) tournaments $T$ such that $i(T) \leq m$. The class $\mathcal {I}_{m}^{< \omega}$ is determined by finitely many obstructions. We give a morphological description of the members of $\mathcal {I}_{1}^{< \omega}$ and a description of the critical obstructions. We give an explicit description of an universal tournament of the class $\mathcal{I}_{m}^{\leq \omega}$.Comment: 6 page

Spectral behavior of some graph and digraph compositions

Let G be a graph of order n the vertices of which are labeled from 1 to n and let $G_1$, · · · ,$G_n$ be n graphs. The graph composition G[$G_1$, · · · ,$G_n$] is the graph obtained by replacing the vertex i of G by the graph Gi and there is an edge between u ∈ $G_i$ and v ∈ $G_j$ if and only if there is an edge between i and j in G. We first consider graph composition G[$K_k$, · · · ,$K_k$] where G is regular and $K_k$ is a complete graph and we establish some links between the spectral characterisation of G and the spectral characterisation of G[$K_k$, · · · ,$K_k$]. We then prove that two non isomorphic graphs G[$G_1$, · · ·$G_n$] where $G_i$ are complete graphs and G is a strict threshold graph or a star are not Laplacian-cospectral, giving rise to a spectral characterization of these graphs. We also consider directed graphs, especially the vertex-critical tournaments without non-trivial acyclic interval which are tournaments of the shape t[$\overrightarrow{C}_{k_1}$, · · · ,$\overrightarrow{C}_{k_m}$], where t is a tournament and $\overrightarrow{C}_{k_i}$ is a circulant tournament. We give conditions to characterise these graphs by their spectrum.Peer Reviewe