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Amsterdam-Zuidoost, Netherlands

Batenburg K.J.,Centrum Wiskunde & Informatica CWI
IEEE transactions on image processing : a publication of the IEEE Signal Processing Society | Year: 2011

In this paper, we present an iterative reconstruction algorithm for discrete tomography, called discrete algebraic reconstruction technique (DART). DART can be applied if the scanned object is known to consist of only a few different compositions, each corresponding to a constant gray value in the reconstruction. Prior knowledge of the gray values for each of the compositions is exploited to steer the current reconstruction towards a reconstruction that contains only these gray values. Based on experiments with both simulated CT data and experimental μCT data, it is shown that DART is capable of computing more accurate reconstructions from a small number of projection images, or from a small angular range, than alternative methods. It is also shown that DART can deal effectively with noisy projection data and that the algorithm is robust with respect to errors in the estimation of the gray values. Source


Laurent M.,Centrum Wiskunde & Informatica CWI | Sun Z.,University of Tilburg
Journal of Global Optimization | Year: 2013

The maximum stable set problem is a well-known NP-hard problem in combinatorial optimization, which can be formulated as the maximization of a quadratic square-free polynomial over the (Boolean) hypercube. We investigate a hierarchy of linear programming relaxations for this problem, based on a result of Handelman showing that a positive polynomial over a polytope with non-empty interior can be represented as conic combination of products of the linear constraints defining the polytope. We relate the rank of Handelman’s hierarchy with structural properties of graphs. In particular we show a relation to fractional clique covers which we use to upper bound the Handelman rank for perfect graphs and determine its exact value in the vertex-transitive case. Moreover we show two upper bounds on the Handelman rank in terms of the (fractional) stability number of the graph and compute the Handelman rank for several classes of graphs including odd cycles and wheels and their complements. We also point out links to several other linear and semidefinite programming hierarchies. © 2013, Springer Science+Business Media New York. Source


Jongmans S.-S.T.Q.,Centrum Wiskunde & Informatica CWI | Santini F.,CNR Institute for Informatics and Telematics | Arbab F.,Centrum Wiskunde & Informatica CWI
Service Oriented Computing and Applications | Year: 2015

Coordination languages, such as Reo, have emerged for the specification and implementation of interaction protocols among concurrent entities, manifested as connectors. In this paper, we describe a theoretical justification and a practical proof-of-concept tool for automatically generating partially distributed, partially centralized implementations of Reo connectors. Such implementations have three performance advantages: faster compilation at build time (compared to a purely centralized approach), reduced latency at run time (compared to a purely distributed approach), and improved parallelism at run time (compared to a purely centralized approach). Our theory relies on the definition of a new product operator on constraint automata (Reo’s formal semantics), which we use to formally justify distributions of disjoint parts of a coordination scheme over different machines according to several possible motivations (e.g., performance, QoS constraints, privacy, resource availability, and network topology). To exemplify our work, in a case study, we show and explain how a generated connector implementation can be executed. © 2015, Springer-Verlag London. Source


Jongmans S.-S.T.Q.,Open University | Jongmans S.-S.T.Q.,Radboud University Nijmegen | Arbab F.,Centrum Wiskunde & Informatica CWI
Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) | Year: 2016

We present PrDK: a development kit for programming protocols. PrDK is based on syntactic separation of process code, presumably written in an existing general-purpose language, and protocol code, written in a domain-specific language with explicit, high-level elements of syntax for programming protocols. PrDK supports two complementary syntaxes (one graphical, one textual) with a common automata-theoretic semantics. As a tool for construction of systems, PrDK consists of syntax editors, a translator, a parser, an interpreter, and a compiler into Java. Performance in the NAS Parallel Benchmarks is promising. © Springer-Verlag Berlin Heidelberg 2016. Source

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