Centrum Wiskunde en Informatica

Amsterdam, Netherlands

Centrum Wiskunde en Informatica

Amsterdam, Netherlands
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Chakraborty S.,Chennai Mathematical Institute | Fischer E.,Technion - Israel Institute of Technology | Gacia-Soriano D.,Centrum Wiskunde en Informatica | Matsliah A.,IBM
Proceedings of the Annual IEEE Conference on Computational Complexity | Year: 2012

We make a step towards characterizing the boolean functions to which isomorphism can be efficiently tested. Specifically, we prove that isomorphism to any boolean function on {0, 1} n with a polynomial number of distinct permutations can be tested with a number of queries that is independent of n. We also show some partial results in the converse direction, and discuss related problems: testing isomorphism up to linear transformations, and testing isomorphism against a uniform (hyper)graph that is given in advance. Our results regarding the latter topic generalize a theorem of Fischer (SICOMP 2005), and in the process we also provide a simpler proof of his original result which avoids the use of Szemerédi's regularity lemma. © 2012 IEEE.

Collins P.,Centrum Wiskunde en Informatica
2009 European Control Conference, ECC 2009 | Year: 2015

In this paper we consider the controllability problem for hybrid systems, namely that of determining the set of states which can be driven into a given target set. We show that given a suitable definition of controllability, we can effectively compute arbitrarily accurate under-approximations to the controllable set using Turing machines. However, due to grazing or sliding along guard sets, we see that it may be able to demonstrate that an initial state can be controlled to the target set, without knowing any trajectory which solves the problem. © 2009 EUCA.

Kemper S.,Centrum Wiskunde en Informatica
Science of Computer Programming | Year: 2012

Component-based software construction relies on suitable models underlying components, and in particular the coordinators which orchestrate component behaviour. Verifying correctness and safety of such systems amounts to model checking the underlying system model. The model checking techniques not only need to be correct (since system sizes increase), but also scalable and efficient. In this paper, we present a SAT-based approach for bounded model checking of Timed Constraint Automata, which permits true concurrency in the timed orchestration of components. We present an embedding of bounded model checking into propositional logic with linear arithmetic. We define a product that is linear in the size of the system, and in this way overcome the state explosion problem to deal with larger systems. To further improve model checking performance, we show how to embed our approach into an extension of counterexample guided abstraction refinement with Craig interpolants. © 2010 Elsevier B.V. All rights reserved.

Rot J.,Leiden University | Bonsangue M.,Centrum Wiskunde en Informatica
Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) | Year: 2014

It was observed by Turi and Plotkin that structural operational semantics can be studied at the level of universal coalgebra, providing specification formats for well-behaved operations on many different types of systems. We extend this framework with non-structural assignment rules which can express, for example, the syntactic format for structural congruences proposed by Mousavi and Reniers. Our main result is that the operational model of such an extended specification is well-behaved, in the sense that bisimilarity is a congruence and that bisimulation-up-to techniques are sound. © 2014 Springer-Verlag.

Herranz J.,Polytechnic University of Catalonia | Hofheinz D.,Karlsruhe Institute of Technology | Kiltz E.,Centrum Wiskunde en Informatica
Information and Computation | Year: 2010

In hybrid public key encryption (PKE), first a key encapsulation mechanism (KEM) is used to fix a random session key that is then fed into a highly efficient data encapsulation mechanism (DEM) to encrypt the actual message. A well-known composition theorem states that if both the KEM and the DEM have a high enough level of security (i.e., security against chosen-ciphertext attacks), then so does the hybrid PKE scheme. It is not known if these strong security requirements on the KEM and DEM are also necessary, nor if such general composition theorems exist for weaker levels of security. Using six different security notions for KEMs, 10 for DEMs, and six for PKE schemes, we completely characterize in this work which combinations lead to a secure hybrid PKE scheme (by proving a composition theorem) and which do not (by providing counterexamples). Furthermore, as an independent result, we revisit and extend prior work on the relations among security notions for KEMs and DEMs. © 2010 Published by Elsevier Inc.

Rot J.,Leiden University | Bonsangue M.,Leiden University | Rutten J.,Centrum Wiskunde en Informatica | Rutten J.,Radboud University Nijmegen
Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) | Year: 2013

Bisimulation-up-to enhances the bisimulation proof method for process equivalence. We present its generalization from labelled transition systems to arbitrary coalgebras, and show that for a large class of systems, enhancements such as bisimulation up to bisimilarity, up to equivalence and up to context are sound proof techniques. This allows for simplified bisimulation proofs for many different types of state-based systems. © 2013 Springer-Verlag Berlin Heidelberg.

Hall R.,Brunel University | Mayhew D.,Victoria University of Wellington | van Zwam S.H.M.,Centrum Wiskunde en Informatica
European Journal of Combinatorics | Year: 2011

In unpublished work, Geelen proved that a matroid is near-regular if and only if it has no minor isomorphic to U2,5, U3,5, F7, F7*, F7-, (F7-)*, AG(2,3)\e, (AG(2,3)\e)*, δT(AG(2,3)\e), or P8. We provide a proof of this characterization. © 2011 Elsevier Ltd.

Hollink V.,Centrum Wiskunde en Informatica | Tsikrika T.,Centrum Wiskunde en Informatica | De Vries A.P.,Centrum Wiskunde en Informatica
Journal of the American Society for Information Science and Technology | Year: 2011

Existing methods for automatically analyzing search logs describe search behavior on the basis of syntactic differences (overlapping terms) between queries. Although these analyses provide valuable insights into the complexity and successfulness of search interactions, they offer a limited interpretation of the observed searching behavior, as they do not consider the semantics of users' queries. In this article we propose a method to exploit semantic information in the form of linked data to enrich search queries so as to determine the semantic types of the queries and the relations between queries that are consecutively entered in a search session. This work provides also an in-depth analysis of the search logs of professional users searching a commercial picture portal. Compared to previous image search log analyses, in particular those of professional users, we consider a much larger dataset. We analyze the logs both in a syntactic way and using the proposed semantic approach and compare the results. Our findings show the benefits of using semantics for search log analysis: the identified types of query modifications cannot be appropriately analyzed by only considering term overlap, since queries related in the most frequent ways do not usually share terms. © 2011 ASIS&T.

He J.,Centrum Wiskunde en Informatica | Hollink V.,Centrum Wiskunde en Informatica | De Vries A.,Centrum Wiskunde en Informatica
SIGIR'12 - Proceedings of the International ACM SIGIR Conference on Research and Development in Information Retrieval | Year: 2012

Result diversification deals with ambiguous or multi-faceted queries by providing documents that cover as many subtopics of a query as possible. Various approaches to subtopic modeling have been proposed. Subtopics have been extracted internally, e.g., from retrieved documents, and externally, e.g., from Web resources such as query logs. Internally modeled subtopics are often implicitly represented, e.g., as latent topics, while externally modeled subtopics are often explicitly represented, e.g., as reformulated queries. We propose a framework that: i)combines both implicitly and explicitly represented subtopics; and ii)allows flexible combination of multiple external resources in a transparent and unified manner. Specifically, we use a random walk based approach to estimate the similarities of the explicit subtopics mined from a number of heterogeneous resources: click logs, anchor text, and web n-grams. We then use these similarities to regularize the latent topics extracted from the top-ranked documents, i.e., the internal (implicit) subtopics. Empirical results show that regularization with explicit subtopics extracted from the right resource leads to improved diversification results, indicating that the proposed regularization with (explicit) external resources forms better (implicit) topic models. Click logs and anchor text are shown to be more effective resources than web n-grams under current experimental settings. Combining resources does not always lead to better results, but achieves a robust performance. This robustness is important for two reasons: it cannot be predicted which resources will be most effective for a given query, and it is not yet known how to reliably determine the optimal model parameters for building implicit topic models. © 2012 ACM.

Pendavingh R.A.,TU Eindhoven | van Zwam S.H.M.,Centrum Wiskunde en Informatica | van Zwam S.H.M.,University of Waterloo
Journal of Combinatorial Theory. Series B | Year: 2010

Let M be a matroid representable over a (partial) field P and B a matrix representable over a sub-partial field P'⊆P. We say that B confines M to P' if, whenever a P-representation matrix A of M has a submatrix B, A is a scaled P'-matrix. We show that, under some conditions on the partial fields, on M, and on B, verifying whether B confines M to P' amounts to a finite check. A corollary of this result is Whittle's Stabilizer Theorem (Whittle, 1999 [34]). A combination of the Confinement Theorem and the Lift Theorem from Pendavingh and Van Zwam (2010) [19] leads to a short proof of Whittle's characterization of the matroids representable over GF(3) and other fields (Whittle, 1997 [33]). We also use a combination of the Confinement Theorem and the Lift Theorem to prove a characterization, in terms of representability over partial fields, of the 3-connected matroids that have k inequivalent representations over GF(5), for k=1,...,6. Additionally we give, for a fixed matroid M, an algebraic construction of a partial field PM and a representation matrix A over PM such that every representation of M over a partial field P is equal to Φ(A) for some homomorphism Φ:PM→P. Using the Confinement Theorem we prove an algebraic analog of the theory of free expansions by Geelen, Oxley, Vertigan, and Whittle (2002) [12]. © 2010 Elsevier Inc.

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