08081 Abstracts Collection -- Data Structures

From February 17th to 22nd 2008, the Dagstuhl Seminar 08081 ``Data Structures\u27\u27 was held in the International Conference and Research Center (IBFI), Schloss Dagstuhl. It brought together 49 researchers from four continents to discuss recent developments concerning data structures in terms of research but also in terms of new technologies that impact how data can be stored, updated, and retrieved. During the seminar a fair number of participants presented their current research. There was discussion of ongoing work, and in addition an open problem session was held. This paper first describes the seminar topics and goals in general, then gives the minutes of the open problem session, and concludes with abstracts of the presentations given during the seminar. Where appropriate and available, links to extended abstracts or full papers are provided

Proceedings of the 8th Cologne-Twente Workshop on Graphs and Combinatorial Optimization

International audienceThe Cologne-Twente Workshop (CTW) on Graphs and Combinatorial Optimization started off as a series of workshops organized bi-annually by either Köln University or Twente University. As its importance grew over time, it re-centered its geographical focus by including northern Italy (CTW04 in Menaggio, on the lake Como and CTW08 in Gargnano, on the Garda lake). This year, CTW (in its eighth edition) will be staged in France for the first time: more precisely in the heart of Paris, at the Conservatoire National d’Arts et Métiers (CNAM), between 2nd and 4th June 2009, by a mixed organizing committee with members from LIX, Ecole Polytechnique and CEDRIC, CNAM

Heterogeneous Self-Reconfiguring Robotics

Self-reconfiguring (SR) robots are modular systems that can autonomously change shape, or reconfigure, for increased versatility and adaptability in unknown environments. In this thesis, we investigate planning and control for systems of non-identical modules, known as heterogeneous SR robots. Although previous approaches rely on module homogeneity as a critical property, we show that the planning complexity of fundamental algorithmic problems in the heterogeneous case is equivalent to that of systems with identical modules. Primarily, we study the problem of how to plan shape changes while considering the placement of specific modules within the structure. We characterize this key challenge in terms of the amount of free space available to the robot and develop a series of decentralized reconfiguration planning algorithms that assume progressively more severe free space constraints and support reconfiguration among obstacles. In addition, we compose our basic planning techniques in different ways to address problems in the related task domains of positioning modules according to function, locomotion among obstacles, self-repair, and recognizing the achievement of distributed goal-states. We also describe the design of a novel simulation environment, implementation results using this simulator, and experimental results in hardware using a planar SR system called the Crystal Robot. These results encourage development of heterogeneous systems. Our algorithms enhance the versatility and adaptability of SR robots by enabling them to use functionally specialized components to match capability, in addition to shape, to the task at hand

Granger Causality Testing in High-Dimensional VARs: a Post-Double-Selection Procedure

We develop an LM test for Granger causality in high-dimensional VAR models based on penalized least squares estimations. To obtain a test retaining the appropriate size after the variable selection done by the lasso, we propose a post-double-selection procedure to partial out effects of nuisance variables and establish its uniform asymptotic validity. We conduct an extensive set of Monte-Carlo simulations that show our tests perform well under different data generating processes, even without sparsity. We apply our testing procedure to find networks of volatility spillovers and we find evidence that causal relationships become clearer in high-dimensional compared to standard low-dimensional VARs

Algorithm design techniques for parameterized graph modification problems

Diese Arbeit beschaeftigt sich mit dem Entwurf parametrisierter Algorithmen fuer Graphmodifikationsprobleme wie Feedback Vertex Set, Multicut in Trees, Cluster Editing und Closest 3-Leaf Powers. Anbei wird die Anwendbarkeit von vier Technicken zur Entwicklung parametrisierter Algorithmen, naemlich, Datenreduktion, Suchbaum, Iterative Kompression und Dynamische Programmierung, fuer solche Graphmodifikationsprobleme untersucht

On some problems in reconstruction

A graph is {\it reconstructible} if it is determined by its {\it deck} of unlabeled subgraphs obtained by deleting one vertex; a {\it card} is one of these subgraphs. The {\it Reconstruction Conjecture} asserts that all graphs with at least three vertices are reconstructible. In Chapter $2$ we consider $k$-deck reconstruction of graphs. The {\it $k$-deck} of a graph is its multiset of $k$-vertex induced subgraphs. We prove a generalization of a result by Bollob\'as concerning the $k$-deck reconstruction of almost all graphs, showing that when $\ell \le (1-\epsilon)\frac{n}{2}$, the probability than an $n$-vertex graph is reconstructible from some $\binom{\ell+1}{2}$ of the graphs in the $(n-\ell)$-deck tends to $1$ as $n$ tends to $\infty$. We determine the smallest $k$ such that all graphs with maximum degree $2$ are $k$-deck reconstructible. We prove for $n\ge 26$ that whether a graph is connected is determined by its $(n-3)$-deck. We prove that if $G$ is a complete $r$-partite graphs, then $G$ is $(r+1)$-deck reconstructible (the same holds for $\overline{G}$). In Chapter $3$ we consider degree-associated reconstruction. An $(n-1)$-vertex induced subgraph accompanied with the degree of the missing vertex is called a {\it dacard}. The {\it degree-associated reconstruction number} of a graph $G$ is the fewest number of dacards needed to determine $G$. We provide a tool for reconstructing some graphs from two dacards. We prove that certain families of trees and disconnected graphs can be reconstructed from two dacards. We also determine the degree-associated reconstruction number for complete multipartite graphs and their complements. For such graphs, we also determine the least $s$ such that {\it every} set of $s$ dacards determine the graph. In Chapter $4$ we consider the reconstruction of matrices from principal submatrices. A $(n-\ell)$-by-$(n-\ell)$ principal submatrix is a submatrix formed by deleting $\ell$ rows and columns symmetrically. The {\it matrix reconstruction threshold} $mrt(\ell)$ is the minimum integer $n_0$ such that for $n\ge n_0$ all $n$-by-$n$ matrices are reconstructible from their deck of $(n-\ell)$-by-$(n-\ell)$ principal submatrices. We prove $mrt(\ell) \leq \frac{2}{\ln 2}\ell^2+3\ell$

Geometric, Algebraic, and Topological Combinatorics

The 2019 Oberwolfach meeting "Geometric, Algebraic and Topological Combinatorics" was organized by Gil Kalai (Jerusalem), Isabella Novik (Seattle), Francisco Santos (Santander), and Volkmar Welker (Marburg). It covered a wide variety of aspects of Discrete Geometry, Algebraic Combinatorics with geometric flavor, and Topological Combinatorics. Some of the highlights of the conference included (1) Karim Adiprasito presented his very recent proof of the $g$-conjecture for spheres (as a talk and as a "Q\&A" evening session) (2) Federico Ardila gave an overview on "The geometry of matroids", including his recent extension with Denham and Huh of previous work of Adiprasito, Huh and Katz

Thermofluid optimisation of turboexpanders for mobile organic Rankine cycle systems

The Organic Rankine Cycle (ORC) is a closed-loop thermodynamic cycle used for low-temperature heat recovery, from sources as diverse as solar, geothermal and industrial processes. Within the ORC, power is produced with the use of a dense-gas vapour expansion in a suitable device and the efficiency of this process is a key influence on cycle efficiency. This thesis therefore investigates the design, optimisation and validation of nozzled radial-inflow turboexpanders, applied to the recovery of vehicle exhaust waste heat. A 1-dimensional meanline performance prediction methodology is developed, capable of modelling the real-gas effects prevalent in dense, organic working fluids. In addition, choking effects within flow passages are explicitly accounted for in both nozzle and rotor, allowing accurate prediction at the high Pressure Ratios typically observed in ORC expanders. A validation case is subsequently presented, tuning loss correlation coefficients to match 6 high Pressure Ratio experimental test cases, reducing average mass flow rate and efficiency errors to 1.33% and 2.04%, respectively. This 1D model is used to create a methodology in which expander optimisation is performed across an entire vehicle duty cycle. A discretised heat exchanger model is developed, capable of modelling multiple fluid phases and real gas effects. A number of simplifications including surrogate models generated by neural network fits, lead to an O(10^3) reduction in computing effort. A case study is performed to quantify benefits to a 11.7 L diesel engine running a 1200-point Non-Road Transient Cycle, resulting in a 21.9% improvement in total cycle energy recovery against a single-point baseline case. The methodologies contained within this thesis are used to design a turboexpander for an industrial research project, focusing on a 15 L diesel Tier 4 engine. Across two generations of design, a combination of computational and experimental methods are applied to produce an expander containing non-radial rotor inlet blading, producing a simulated power output of 12.4 kW at the design point. Finally, in order to facilitate validation of real-gas Computational Fluid Dynamics (CFD) simulations and better understand the underlying flow physics, a blowdown facility is constructed for working fluid r1233zd(E), centred around a converging-diverging test section producing a Mach 2 expansion at the exit plane. Comparison of CFD and centreline pressure measurements for both nitrogen and refrigerant produced maximum errors in Pressure Ratio of 5.7% and 12.1% respectively, suggesting an overestimation by computational methods in predicting expander output power. The methodologies, designs and experimental results contained within this thesis provide improvements to the modelling, optimisation and validation of dense-gas turboexpanders, with the aim of improving design processes and aiding the implementation of ORC for vehicle waste heat recovery.Open Acces