Clique-width: When Hard Does Not Mean Impossible

In recent years, the parameterized complexity approach has lead to the introduction of many new algorithms and frameworks on graphs and digraphs of bounded clique-width and, equivalently, rank-width. However, despite intensive work on the subject, there still exist well-established hard problems where neither a parameterized algorithm nor a theoretical obstacle to its existence are known. Our article is interested mainly in the digraph case, targeting the well-known Minimum Leaf Out-Branching (cf. also Minimum Leaf Spanning Tree) and Edge Disjoint Paths problems on digraphs of bounded clique-width with non-standard new approaches. The first part of the article deals with the Minimum Leaf Out-Branching problem and introduces a novel XP-time algorithm wrt. clique-width. We remark that this problem is known to be W[2]-hard, and that our algorithm does not resemble any of the previously published attempts solving special cases of it such as the Hamiltonian Path. The second part then looks at the Edge Disjoint Paths problem (both on graphs and digraphs) from a different perspective -- rather surprisingly showing that this problem has a definition in the MSO_1 logic of graphs. The linear-time FPT algorithm wrt. clique-width then follows as a direct consequence

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

Formal verification of Sequential Systems with Infinitely Many States

In recent years, model checking algorithms for verification of infinite-state systems were deeply studied and applied to practical problems. We show, how to use the algorithms for pushdown systems and various modal logics of [4] for verification of Java programs. The process of mechanical abstract model generation is described, and a prototype tool called JAVACHECK is implemented to verify our concept. We also present some examples of properties which can be verified using our approach

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 treewidth. Finally, we discuss some implications and future work

OpenMP for Java

The first part of this report describes the current state of the JOMP, a definition and implementation of a set of directives and library methods for shared memory parallel programming in Java