Jan Obdržálek

DAG-width: connectivity measure for directed graphs

Tree-width is a very useful connectivity measure for undirected graphs. We propose a new definition, called DAG-width, for directed graphs which measures how close a graph is to a directed acyclic graph. In addition we define a cops-and-robber game and show that this game characterises exactly the class of graphs of bounded DAG-width. A comparison of DAG-width with tree-width and directed tree-width follows. Finally we show that NP-complete problems can be solved in polynomial time on graphs of bounded DAG-width.

Fast Mu-calculus Model Checking when Tree-width is Bounded

We show that the model checking problem for μ-calculus on graphs of bounded tree-width can be solved in time linear in the size of the system. The result is presented by first showing a related result: the winner in a parity game on a graph of bounded tree-width can be decided in polynomial time. The given algorithm is then modified to obtain a new algorithm for μ-calculus model checking. One possible use of this algorithm may be software verification, since control flow graphs of programs written in high-level languages are usually of bounded tree-width. Finally, we discuss some implications and future work.

Clique-width and parity games

The question of the exact complexity of solving parity games is one of the major open problems in system verification, as it is equivalent to the problem of model-checking the modal µ-calculus. The known upper bound is NP∩co-NP, but no polynomial algorithm is known. It was shown that on tree-like graphs (of bounded tree-width and DAG-width) a polynomial-time algorithm does exist. Here we present a polynomial-time algorithm for parity games on graphs of bounded clique-width (class of graphs containing e.g. complete bipartite graphs and cliques), thus completing the picture. This also extends the tree-width result, as graphs of bounded tree-width are a subclass of graphs of bounded clique-width. The algorithm works in a different way to the tree-width case and relies heavily on an interesting structural property of parity games.

On Digraph Width Measures in Parameterized Algorithmics

In contrast to undirected width measures (such as tree-width or clique-width), which have provided many important algorithmic applications, analogous measures for digraphs such as DAG-width or Kelly-width do not seem so successful. Several recent papers, e.g. those of Kreutzer---Ordyniak, Dankelmann---Gutin---Kim, or Lampis---Kaouri---Mitsou, have given some evidence for this. We support this direction by showing that many quite different problems remain hard even on graph classes that are restricted very beyond simply having small DAG-width. To this end, we introduce new measures K-width and DAG-depth. On the positive side, we also note that taking Kantes directed generalization of rank-width as a parameter makes many problems fixed parameter tractable.

Kernelization Using Structural Parameters on Sparse Graph Classes

Meta-theorems for polynomial (linear) kernels have been the subject of intensive research in parameterized complexity. Heretofore, there were meta-theorems for linear kernels on graphs of bounded genus, H-minor-free graphs, and H-topological-minor-free graphs. To the best of our knowledge, there are no known meta-theorems for kernels for any of the larger sparse graph classes: graphs of bounded expansion, locally bounded expansion, and nowhere dense graphs. In this paper we prove meta-theorems for these three graph classes. More specifically, we show that graph problems that have finite integer index (FII) admit linear kernels on hereditary graphs of bounded expansion when parameterized by the size of a modulator to constant-treedepth graphs. For hereditary graph classes of locally bounded expansion, our result yields a quadratic kernel and for hereditary nowhere dense graphs, a polynomial kernel. While our parameter may seem rather strong, a linear kernel result on graphs of bounded expansion with a weaker parameter would for some problems violate known lower bounds. Moreover, we use a relaxed notion of FII which allows us to prove linear kernels for problems such as Longest Path/Cycle and Exact s,t-Path which do not have FII in general graphs.

Digraph width measures in parameterized algorithmics

Abstract In contrast to undirected width measures such as tree-width, which have provided many important algorithmic applications, analogous measures for digraphs such as directed tree-width or DAG-width do not seem so successful. Several recent papers have given some evidence on the negative side. We confirm and consolidate this overall picture by thoroughly and exhaustively studying the complexity of a range of directed problems with respect to various parameters, and by showing that they often remain NP-hard even on graph classes that are restricted very beyond having small DAG-width. On the positive side, it turns out that clique-width (of digraphs) performs much better on virtually all considered problems, from the parameterized complexity point of view.

Better Algorithms for Satisfiability Problems for Formulas of Bounded Rank-width

We provide a parameterized algorithm for the propositional model counting problem #SAT, the runtime of which has a single-exponential dependency on the rank-width of the signed graph of a formula. That is, our algorithm runs in time

An OpenMP‐like interface for parallel programming in Java

\cal{O}t^3 \cdot 2^{3tt+1/2} \cdot \vert\phi\vert

Clique-width: When Hard Does Not Mean Impossible

for a width-t rank-decomposition of the input φ, and can be of practical interest for small values of rank-width. Previously, analogical algorithms have been known --e.g. [Fischer, Makowsky, and Ravve] --with a single-exponential dependency on the clique-width k of the signed graph of a formula with a given k-expression. Our algorithm presents an exponential runtime improvement over the worst-case scenario of the previous one, since clique-width reaches up to exponentially higher values than rankwidth. We also provide an algorithm for the MAX-SAT problem along the same lines.

Are there any good digraph width measures

This paper describes the definition and implementation of an OpenMP‐like set of directives and library routines for shared memory parallel programming in Java. A specification of the directives and routines is proposed and discussed. A prototype implementation, consisting of a compiler and a runtime library, both written entirely in Java, is presented, which implements most of the proposed specification. Some preliminary performance results are reported. Copyright