THOMASSÉ , Stéphan

Answering a question of Geelen, Gerards, Robertson and Whittle, we prove that the branchwidth of a bridgeless graph is equal to the branch- width of its cycle matroid. Our proof is based on branch-decompositions of hypergraph.

A {\it $(p,1)$-total labelling} of a graph $G=(V,E)$ is a total coloring $L$ from $V\cup E$ into $\{0,\dots ,l\}$ such that $|L(v)-L(e)|\geq p$ whenever an edge $e$ is incident to a vertex $v$. The minimum $l$ for which $G$ admits a $(p,1)$-total labelling is denoted by $\lambda_p(G)$. The case $p=1$ corresponds to the usual notion of total...

A {\it $(p,1)$-total labelling} of a graph $G=(V,E)$ is a total coloring $L$ from $V\cup E$ into $\{0,\dots ,l\}$ such that $|L(v)-L(e)|\geq p$ whenever an edge $e$ is incident to a vertex $v$. The minimum $l$ for which $G$ admits a $(p,1)$-total labelling is denoted by $\lambda_p(G)$. The case $p=1$ corresponds to the usual notion of total...

Hoàng-Reed conjecture asserts that every digraph $D$ has a collection $\cal C$ of circuits $C_1,\dots,C_{\delta ^+}$, where $\delta ^+$ is the minimum outdegree of $D$, such that the circuits of $\cal C$ have a forest-like structure. Formally, $|V(C_i)\cap (V(C_1)\cup \dots \cup V(C_{i-1}))|\leq 1$, for all $i=2,\dots ,\delta^+$. We verify this...

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Hoàng-Reed conjecture asserts that every digraph $D$ has a collection $\cal C$ of circuits $C_1,\dots,C_{\delta ^+}$, where $\delta ^+$ is the minimum outdegree of $D$, such that the circuits of $\cal C$ have a forest-like structure. Formally, $|V(C_i)\cap (V(C_1)\cup \dots \cup V(C_{i-1}))|\leq 1$, for all $i=2,\dots ,\delta^+$. We verify this...

A very nice result of Bárány and Lehel asserts that every finite subset X or can be covered by X-boxes (i.e. each box has two antipodal points in X). As shown by Gyárfás and Pálvőlgyi this result would follow from the following conjecture: If a tournament admits a partition of its arc set into k quasi-orders, then its domination number is...

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We show that every oriented path of order n>=4 with two blocks is contained in every n-chromatic digraph.

We show that every oriented path of order $n\geq4$ with two blocks is contained in every $n$-chromatic digraph.

A digraph is {\it $m$-labelled} if every arcs is labelled by an integer in $\{1, \dots ,m\}$. Motivated by wavelength assignment for multicasts in optical star networks, we study {\it $n$-fiber colourings} of labelled digraph which are colourings of the arcs of $D$ such that at each vertex $v$, for each colour in $\lambda$,...

In a directed graph, a star is an arborescence with at least one arc, in which the root dominates all the other vertices. A galaxy is a vertex-disjoint union of stars. In this paper, we consider the Spanning Galaxy problem of deciding whether a digraph D has a spanning galaxy or not. We show that although this problem is NP-complete (even...

Adapting the method introduced in Graph Minors X, we propose a new proof of the duality between the bramble number of a graph and its tree-width. Our approach is based on a new definition of submodularity on partition functions which naturally extends the usual one on set functions. The proof does not rely on Menger's theorem, and thus...

A digraph is {\it $m$-labelled} if every arcs is labelled by an integer in $\{1, \dots ,m\}$. Motivated by wavelength assignment for multicasts in optical star networks, we study {\it $n$-fiber colourings} of labelled digraph which are colourings of the arcs of $D$ such that at each vertex $v$, for each colour $\lambda$,...

A star is an arborescence in which the root dominates all the other vertices. A galaxy is a vertex-disjoint union of stars. The directed star arboricity of a digraph D, denoted by dst(D), is the minimum number of galaxies needed to cover A(D). In this paper, we show that dst(D)less-than-or-equals, slantΔ(D)+1 and that if D is acyclic then...

In 2006, Barát and Thomassen conjectured that there is a function f such that, for every fixed tree T with t edges, every f(t)-edge-connected graph with its number of edges divisible by t has a partition of its edges into copies of T. This conjecture was recently verified by the current authors and Merker. We here further focus on the path case...

Art gallery problems have been extensively studied over the last decade and have found different type of applications. Normally the number of sides of a polygon or the general shape of the polygon is used as a measure of the complexity of the problem. In this paper we explore another measure of complexity, namely, the number of guards required...

The Barát-Thomassen conjecture asserts that for every tree T on m edges, there exists a constant k T such that every k T-edge-connected graph with size divisible by m can be edge-decomposed into copies of T. So far this conjecture has only been verified when T is a path or when T has diameter at most 4. Here we prove the full statement of the...

Let $f(k)$ be the smallest integer such that every $f(k)$-chromatic digraph contains every oriented tree of order $k$. Burr proved $f(k)\leq (k-1)^2$ in general, and he conjectured $f(k)=2k-2$. Burr also proved that every $(8k-7)$-chromatic digraph contains every antidirected tree. We improve both of Burr's bounds. We show that $f(k)\leq...

In 1985, Mader conjectured the existence of a function f such that every digraph with minimum out-degree at least f (k) contains a subdivision of the transitive tournament of order k. This conjecture is still completely open, as the existence of f (5) remains unknown. In this paper, we show that if D is an oriented path, or an in-arborescence...

In 1985, Mader conjectured the existence of a function f such that every digraph with minimum out-degree at least f (k) contains a subdivision of the transitive tournament of order k. This conjecture is still completely open, as the existence of f (5) remains unknown. In this paper, we show that if D is an oriented path, or an in-arborescence...

Let f (k) be the smallest integer such that every f (k)-chromatic digraph contains every oriented tree of order k. Burr proved that f (k) ≤ (k − 1)^2 and conjectured f (k) = 2n − 2. In this paper, we give some sufficient conditions for an n-chromatic digraphs to contains some oriented tree. In particular, we show that every acyclic n-chromatic...