Centrum Wiskunde & Informatica CWI

Amsterdam, Netherlands

Centrum Wiskunde & Informatica CWI

Amsterdam, Netherlands
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Hoogland J.,Centrum Wiskunde & Informatica CWI | La Poutre H.,Centrum Wiskunde & Informatica CWI | La Poutre H.,Technical University of Delft
Lecture Notes in Business Information Processing | Year: 2017

The Power TAC is a competition-based simulation of an electricity market. The goal of the competition is to test retailer (broker) strategies in a competitive environment. Participants create broker agents that trade electricity. In this paper we describe our broker, which we created as a participant of the 2014 Power TAC competition. We describe the strategies for two main components of the game: the tariff market and the wholesale market.We also discuss the performance of our broker in the competition, where we were second in the final ranking. © Springer International Publishing AG 2017.


Azadbakht K.,Centrum Wiskunde & Informatica CWI | Bezirgiannis N.,Centrum Wiskunde & Informatica CWI | de Boer F.S.,Centrum Wiskunde & Informatica CWI
Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) | Year: 2017

Generation of social networks using Preferential Attachment (PA) mechanism is proposed in the Barabasi-Albert model. In this mechanism, new nodes are introduced to the network sequentially and they attach to the existing nodes preferentially where the preference can be based on the degree of the existing nodes. PA is a classical model with a natural intuition, great explanatory power and interesting mathematical properties. Some of these properties only appear in large-scale networks. However generation of such extra-large networks can be challenging due to memory limitations. In this paper, we investigate a distributedmemory approach for PA-based network generation which is scalable and which avoids low-level synchronization mechanisms thanks to utilizing a powerful programming model and proper programming constructs. © Springer International Publishing AG 2017.


Briet J.,Centrum Wiskunde & Informatica CWI | Zuiddam J.,Centrum Wiskunde & Informatica CWI
Quantum Information and Computation | Year: 2017

After Bob sends Alice a bit, she responds with a lengthy reply. At the cost of a factor of two in the total communication, Alice could just as well have given Bob her two possible replies at once without listening to him at all, and have him select which one applies. Motivated by a conjecture stating that this form of “round elimination” is impossible in exact quantum communication complexity, we study the orthogonal rank and a symmetric variant thereof for a certain family of Cayley graphs. The orthogonal rank of a graph is the smallest number d for which one can label each vertex with a nonzero d-dimensional complex vector such that adjacent vertices receive orthogonal vectors. We show an exp(n) lower bound on the orthogonal rank of the graph on {0, 1}n in which two strings are adjacent if they have Hamming distance at least n/2. In combination with previous work, this implies an affirmative answer to the above conjecture. © Rinton Press.


Kleer P.,Centrum Wiskunde & Informatica CWI | Schafer G.,Centrum Wiskunde & Informatica CWI | Schafer G.,VU University Amsterdam
Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) | Year: 2017

We introduce a new model of congestion games that cap-tures several extensions of the classical congestion game introduced by Rosenthal in 1973. The idea here is to parameterize both the perceived cost of each player and the social cost function of the system designer. Intuitively, each player perceives the load induced by the other players by an extent of ρ ≥ 0, while the system designer estimates that each player perceives the load of all others by an extent of σ ≥ 0. For specific choices of ρ and σ, we obtain extensions such as altruistic player behav-ior, risk sensitive players and the imposition of taxes on the resources. We derive tight bounds on the price of anarchy and the price of stability for a large range of parameters. Our bounds provide a complete picture of the inefficiency of equilibria for these games. As a result, we obtain tight bounds on the price of anarchy and the price of stability for the above mentioned extensions. Our results also reveal how one should “design” the cost functions of the players in order to reduce the price of anarchy. Somewhat counterintuitively, if each player cares about all other players to the extent of ρ = 0.625 (instead of 1 in the standard setting) the price of anarchy reduces from 2.5 to 2.155 and this is best possible. © Springer International Publishing AG 2017.


Azadbakht K.,Centrum Wiskunde & Informatica CWI | Bezirgiannis N.,Centrum Wiskunde & Informatica CWI | de Boer F.S.,Centrum Wiskunde & Informatica CWI
Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) | Year: 2017

Many modern distributed software applications require a continuous interaction between their components exploiting streaming data from the server to the client. The Abstract Behavioral Specification (ABS) language has been developed for the modeling and analysis of distributed systems. In ABS, concurrent objects communicate by calling each other’s methods asynchronously. Return values are communicated asynchronously too via the return statement and so-called futures. In this paper, we extend the basic ABS model of asynchronous method invocation and return in order to support the streaming of data. We introduce the notion of a “Future-based Data Stream” to extend the ABS. The application of this notion and its impact on performance are illustrated by means of a case study in the domain of social networks simulation. © IFIP International Federation for Information Processing 2017.


Berkhout J.,Centrum Wiskunde & Informatica CWI | Heidergott B.F.,VU University Amsterdam
Discrete Event Dynamic Systems: Theory and Applications | Year: 2017

The basis of Google’s acclaimed PageRank is an artificial mixing of the Markov chain representing the connectivity structure of the network under study with a maximally connected network where every node is connected to every other node. The rate with which the original network is mixed with the strongly connected one is called damping factor. The choice of the damping factor can influence the ranking of the nodes. As we show in this paper, the ranks of transient nodes, i.e., nodes not belonging to a strongly connected component without outgoing links in the original network, tend to zero as the damping factor increases. In this paper we develop a new methodology for obtaining a meaningful ranking of nodes without having to resort to mixing the network with an artificial one. Our new ranking relies on an adjusted definition of the ergodic projector of the Markov chain representing the original network. We will show how the new ergodic projector leads to a more structural way of ranking (transient) nodes. Numerical examples are provided to illustrate the impact of this new ranking approach. © 2017 Springer Science+Business Media New York


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.


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.


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.


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.

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