Ewald Sums for One Dimension

We derive analytic solutions for the potential and field in a one-dimensional system of masses or charges with periodic boundary conditions, in other words Ewald sums for one dimension. We also provide a set of tools for exploring the system evolution and show that it's possible to construct an efficient algorithm for carrying out simulations. In the cosmological setting we show that two approaches for satisfying periodic boundary conditions, one overly specified and the other completely general, provide a nearly identical clustering evolution until the number of clusters becomes small, at which time the influence of any size-dependent boundary cannot be ignored. Finally we compare the results with other recent work with the hope of providing clarification over differences these issues have induced. We explain that modern formulations of physics require a well defined potential which is not available if the forces are screened directly.Comment: 2 figures added references expanded discussion of algorithm corrected figures added discussion of screened forc

On the voting power of an alliance and the subsequent power of its members

Even, and in fact chiefly, if two or more players in a voting game have on a binary issue independent opinions, they may have interest to form a single voting alliance giving an average gain of influence for all of them. Here, assuming the usual independence of votes, we first study the alliance voting power and obtain new results in the so-called asymptotic limit for which the number of players is large enough and the alliance weight remains a small fraction of the total of the weights. Then, we propose to replace the voting game inside the alliance by a random game which allows new possibilities. The validity of the asymptotic limit and the possibility of new alliances are examined by considering the decision process in the Council of Ministers of the European Union.

A distributed-memory package for dense Hierarchically Semi-Separable matrix computations using randomization

We present a distributed-memory library for computations with dense structured matrices. A matrix is considered structured if its off-diagonal blocks can be approximated by a rank-deficient matrix with low numerical rank. Here, we use Hierarchically Semi-Separable representations (HSS). Such matrices appear in many applications, e.g., finite element methods, boundary element methods, etc. Exploiting this structure allows for fast solution of linear systems and/or fast computation of matrix-vector products, which are the two main building blocks of matrix computations. The compression algorithm that we use, that computes the HSS form of an input dense matrix, relies on randomized sampling with a novel adaptive sampling mechanism. We discuss the parallelization of this algorithm and also present the parallelization of structured matrix-vector product, structured factorization and solution routines. The efficiency of the approach is demonstrated on large problems from different academic and industrial applications, on up to 8,000 cores. This work is part of a more global effort, the STRUMPACK (STRUctured Matrices PACKage) software package for computations with sparse and dense structured matrices. Hence, although useful on their own right, the routines also represent a step in the direction of a distributed-memory sparse solver

Interactivité et compatibilité cognitive dans les systèmes hypermédias

Le but de cet article est de montrer que l'efficacité des systèmes hypermedias comme outils d'apprentissage dépend de leur compatibilité avec les processus et les représentations cognitives des utilisateurs. Dans un premier temps, la compatibilité cognitive est définie dans le cadre général de l'interaction sujet-ordinateur. Puis cette notion est précisée dans le cas particulier de la lecture-compréhension des documents hypermedias. A partir des recherches psychologiques sur la compréhension, trois facteurs de compatibilité cognitive sont identifiés: la structure rhétorique globale de l'hyperdocument, la cohérence interne et l'intégration multimodale. On en conclut que la conception d'hyperdocuments pour l'apprentissage doit s'inspirer systématiquement, mais non exclusivement, des modèles cognitifs du traitement de l'information complexe.The aim of this article is to demonstrate that the efficacy of hypermedia systems as learning tools depends on their compatibility with processes and cognitive representations of users. Firstly, the author defines cognitive compatibility with the general frame of subject computer interaction. This concept is then clarified using the specific case of reading comprehension of hypermedia documents. Based on research on the psychology of comprehension, three cognitive compatibility factors are identified: the global rhetorical structure of the hyperdocument, the internal coherence, and multimodel integration. The author concludes that the conceptualization of hypertext for purposes of learning should be based systematically, but not exclusively, on cognitive models involved in processing complex information.El objetivo de este articulo es de mostrar que la eficacia de los sistemas hipermedia como instrumentes de aprendizaje, dependen de su compatibilidad con los procesos y las representaciones cognitivas de los usuarios. Primero, la compatibilidad cognitiva es defînida dentro del marco general de la interaction sujeto-computador. Luego, esta nociôn es precisada para el caso particular de la lectura-comprehension de documentos hipermedios. A partir de estudios psicologicos realizados sobre la comprehension, se han podido identificar très factores de compatibilidad cognitiva: la estructura retorica global del hiperdocumento, la coherencia interna y la integracion multimodal. Se conclure que la concepcion de hiperdocumentos orientados al aprendizaje debe inspirarse sistematicamente aunque no exclusivamente en los modelos cognitivos para el tratamiento de informaciones complejas.In diesem Artikel wird versucht zu zeigen, dass die Wirksamkeit von Hypermediensystemen als Mittel der Lernhilfe von ihrer Kompatibilitàt mit den kognitiven Vorgângen und Vorstellungen der Benutzer abhângt. Zunâchst wird die kognitive Kompatibilitàt im allgemeinen Rahmen der Benutzer-Computer-Interaktion definiert. Dann wird dieser Begriff genauer ins Auge gefasst im besonderen Fall des Lesens/ Verstehens von Hypermediendokumenten. Ausgehend von psychologischen Forschungsarbeiten liber das Verstàndnis werden drei Faktoren der kognitiven Kompatibilitàt aufgestellt : die allgemeine rhetorische Struktur des Hyperdokumentes, die innere Kohârenz und die multimodale Integration. Es wird daraus geschlossen, dass die Konzeption von Hyperdokumenten als Lernhilfe systematisch, wenn auch nicht ausschliefilich, auf die kognitiven Modelle der Verarbeitung komplexer Information zuriickgreifen muss

Shadowing effects for continuum and discrete deposition models

We study the dynamical evolution of the deposition interface using both discrete and continuous models for which shadowing effects are important. We explain why continuous and discrete models implying both only shadowing deposition do not give the same result and propose a continuous model which allow to recover the result of the discrete one exhibiting a strong columnar morphology