Saeed Akhoondian Amiri

A Local Constant Factor MDS Approximation for Bounded Genus Graphs

The Minimum Dominating Set (MDS) problem is not only one of the most fundamental problems in distributed computing, it is also one of the most challenging ones. While it is well-known that minimum dominating sets cannot be approximated locally on general graphs, over the last years, several breakthroughs have been made on computing local approximations on sparse graphs. This paper presents a deterministic and local constant factor approximation for minimum dominating sets on bounded genus graphs, a large family of sparse graphs. Our main technical contribution is a new analysis of a slightly modified variant of an existing algorithm by Lenzen et al. Interestingly, unlike existing proofs for planar graphs, our analysis does not rely on direct topological arguments. We believe that our techniques can be useful for the study of local problems on sparse graphs beyond the scope of this paper.

Transiently Consistent SDN Updates: Being Greedy is Hard

The software-defined networking paradigm introduces interesting opportunities to operate networks in a more flexible yet formally verifiable manner. Despite the logically centralized control, however, a Software-Defined Network (SDN) is still a distributed system, with inherent delays between the switches and the controller. Especially the problem of changing network configurations in a consistent manner, also known as the consistent network update problem, has received much attention over the last years. This paper revisits the problem of how to update an SDN in a transiently consistent, loop-free manner. First, we rigorously prove that computing a maximum (“greedy”) loop-free network update is generally NP-hard; this result has implications for the classic maximum acyclic subgraph problem (the dual feedback arc set problem) as well. Second, we show that for special problem instances, fast and good approximation algorithms exist.

Graph Searching Games and Width Measures for Directed Graphs

In cops and robber games a number of cops tries to capture a robber in a graph. A variant of these games on undirected graphs characterises tree width by the least number of cops needed to win. We consider cops and robber games on digraphs and width measures (such as DAG-width, directed tree width or D-width) corresponding to them. All of them generalise tree width and the game characterising it. For the DAG-width game we prove that the problem to decide the minimal number of cops required to capture the robber (which is the same as deciding DAG-width), is PSPACE-complete, in contrast to most other similar games. We also show that the cop-monotonicity cost for directed tree width games cannot be bounded by any function. As a consequence, D-width is not bounded in directed tree width, refuting a conjecture by Safari. A large number of directed width measures generalising tree width has been proposed in the literature. However, only very little was known about the relation between them, in particular about whether classes of digraphs of bounded width in one measure have bounded width in another. In this paper we establish an almost complete order among the most prominent width measures with respect to mutual boundedness.

Distributed Domination on Graph Classes of Bounded Expansion

\noindent We provide a new constant factor approximation algorithm for the (connected) \mboxdistance- r dominating set problem on graph classes of bounded expansion. Classes of bounded expansion include many familiar classes of sparse graphs such as planar graphs and graphs with excluded (topological) minors, and notably, these classes form the most general subgraph closed classes of graphs for which a sequential constant factor approximation algorithm for the distance- r dominating set problem is currently known. Our algorithm can be implemented in the \congestbc model of distributed computing and uses

Walking Through Waypoints

Øof(r^2 łog n)

Vertex Disjoint Paths in Upward Planar Graphs

communication rounds. % Our techniques, which may be of independent interest, are based on a distributed computation of sparse neighborhood covers of small radius on bounded expansion classes. We show how to compute an r -neighborhood cover of radius~

DAG-width is PSPACE-complete☆

2r

Charting the Algorithmic Complexity of Waypoint Routing

and overlap

Routing with Congestion in Acyclic Digraphs

f(r)

Congestion-Free Rerouting of Flows on DAGs.

on every class of bounded expansion in