Dagstuhl Reports : Volume 1, Issue 2, February 2011

Online Privacy: Towards Informational Self-Determination on the Internet (Dagstuhl Perspectives Workshop 11061) : Simone Fischer-Hübner, Chris Hoofnagle, Kai Rannenberg, Michael Waidner, Ioannis Krontiris and Michael Marhöfer Self-Repairing Programs (Dagstuhl Seminar 11062) : Mauro Pezzé, Martin C. Rinard, Westley Weimer and Andreas Zeller Theory and Applications of Graph Searching Problems (Dagstuhl Seminar 11071) : Fedor V. Fomin, Pierre Fraigniaud, Stephan Kreutzer and Dimitrios M. Thilikos Combinatorial and Algorithmic Aspects of Sequence Processing (Dagstuhl Seminar 11081) : Maxime Crochemore, Lila Kari, Mehryar Mohri and Dirk Nowotka Packing and Scheduling Algorithms for Information and Communication Services (Dagstuhl Seminar 11091) Klaus Jansen, Claire Mathieu, Hadas Shachnai and Neal E. Youn

Dagstuhl Annual Report January - December 2011

The International Conference and Research Center for Computer Science is a non-profit organization. Its objective is to promote world-class research in computer science and to host research seminars which enable new ideas to be showcased, problems to be discussed and the course to be set for future development in this field. The work being done to run this informatics center is documented in this report for the business year 2011

Spectral Complexity of Directed Graphs and Application to Structural Decomposition

We introduce a new measure of complexity (called spectral complexity) for directed graphs. We start with splitting of the directed graph into its recurrent and non-recurrent parts. We define the spectral complexity metric in terms of the spectrum of the recurrence matrix (associated with the reccurent part of the graph) and the Wasserstein distance. We show that the total complexity of the graph can then be defined in terms of the spectral complexity, complexities of individual components and edge weights. The essential property of the spectral complexity metric is that it accounts for directed cycles in the graph. In engineered and software systems, such cycles give rise to sub-system interdependencies and increase risk for unintended consequences through positive feedback loops, instabilities, and infinite execution loops in software. In addition, we present a structural decomposition technique that identifies such cycles using a spectral technique. We show that this decomposition complements the well-known spectral decomposition analysis based on the Fiedler vector. We provide several examples of computation of spectral and total complexities, including the demonstration that the complexity increases monotonically with the average degree of a random graph. We also provide an example of spectral complexity computation for the architecture of a realistic fixed wing aircraft system.Comment: We added new theoretical results in Section 2 and introduced a new section 2.2 devoted to intuitive and physical explanations of the concepts from the pape

Dagstuhl News January - December 2011

"Dagstuhl News" is a publication edited especially for the members of the Foundation "Informatikzentrum Schloss Dagstuhl" to thank them for their support. The News give a summary of the scientific work being done in Dagstuhl. Each Dagstuhl Seminar is presented by a small abstract describing the contents and scientific highlights of the seminar as well as the perspectives or challenges of the research topic

Foreword: Special Issue on Theory and Applications of Graph Searching Problems

International audienceIn a Graph Searching Game, one part is a set of escaping mobile entities, called evaders (or fugitives), that hide in a graph representing a network, and the other part is a number of chasing agents, called searchers (or pursuers), that move systematically in the graph. The game may vary significantly according to the capabilities of the evaders and the pursuers in terms of relative speed, sensor capabilities, visibility, etc. The objective of the game is to capture the evaders in an optimal way, where the notion of optimality itself admits varying interpretations. Alternatively, such games are known as pursuit- evasion games or cops and robbers games. Graph searching revealed the need to express in a formal mathematical way intuitive concepts such as avoidance, surrounding, sense of direction, hiding, persecution, threatening, collaboration, and coalition. There are many variants of graph searching studied in the literature, which are either application driven, i.e. motivated by problems in practice, or are inspired by foundational issues in Computer Science, Discrete Mathematics, and Artificial Intelligence. They are related to diverse topics such as Information Seeking, Robot motion planning, Database Theory, Logic, Distributed Computing, Models of computation, and Network security. The workshop series GRASTA, GRAph Searching, Theory and Applications was established as the main forum on the area of Graph Searching and gathers the widest possible variety of related disciplines. It has already held in Anogia, Crete, Greece (2006), Praia de Redonda, Ceara, Brazil (2008), and Valtice chateau, Valtice, Czech Republic (2009). This volume is a follow up of the fourth event of the series, namely, the 4th Workshop on GRAph Searching, Theory and Applications (GRASTA 2011) that took place during Dagstuhl Seminar 11071, from February 13, 2011 to February 18, 2012. While most of the submissions came from the participants of the workshop, there were also many from other contributors. There were, 19 submissions in total. All papers have been carefully reviewed according to the highest standards of Theoretical Computer Science and, among them, 11 have finally been accepted. The material appearing in this issue comprises a wide variety of topics including, Parity games, (hyper)Graph parameters, Graph exploration, Online algorithms, Competitive analysis, Sensor networks, Cleaning games, Chip firing problems, and Random walks. Finally, we are in the happy position to dedicate this issue to the 60th birthday of Professor Lefteris M. Kirousis. His insight and early vision on Graph Searching has been a source of inspiration for many of us