169 research outputs found
On Distance Coloring
Call a connected undirected graph (d,c)-colorable if there is a vertex coloring using at most c colors such that no two vertices of distance d or less have the same color. It is well known that (1,2)-colorability is decidable in linear time, but (1,c)-colorability for c greater than or equal to 3 is NP-complete. Sharp (2007) shows that for fixed d greater than or equal to 2, the (d,c)-colorability problem is solvable in linear time for c less than or equal to 3d/2 and NP-complete otherwise. In this note we give an alternative construction that improves the upper time bound as a function of d for the case c less than or equal to 3d/2. The construction entails a generalization of the notion of tree decomposition and bounded treewidth (Robertson and Seymour 1986) to arbitrary overlay graphs, not just trees, which may be of independent interest
Embedded graph 3-coloring and flows
A graph drawn in a surface is a near-quadrangulation if the sum of the
lengths of the faces different from 4-faces is bounded by a fixed constant. We
leverage duality between colorings and flows to design an efficient algorithm
for 3-precoloring-extension in near-quadrangulations of orientable surfaces.
Furthermore, we use this duality to strengthen previously known sufficient
conditions for 3-colorability of triangle-free graphs drawn in orientable
surfaces.Comment: 53 pages, 15 figure
Three-coloring triangle-free graphs on surfaces III. Graphs of girth five
We show that the size of a 4-critical graph of girth at least five is bounded
by a linear function of its genus. This strengthens the previous bound on the
size of such graphs given by Thomassen. It also serves as the basic case for
the description of the structure of 4-critical triangle-free graphs embedded in
a fixed surface, presented in a future paper of this series.Comment: 53 pages, 7 figures; updated according to referee remark
Algorithmic Graph Theory
The main focus of this workshop was on mathematical techniques needed for the development of efficient solutions and algorithms for computationally difficult graph problems. The techniques studied at the workshhop included: the probabilistic method and randomized algorithms, approximation and optimization, structured families of graphs and approximation algorithms for large problems. The workshop Algorithmic Graph Theory was attended by 46 participants, many of them being young researchers. In 15 survey talks an overview of recent developments in Algorithmic Graph Theory was given. These talks were supplemented by 10 shorter talks and by two special sessions
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