1,720,963 research outputs found
Gibbs and non-Gibbs aspects of continuous spin models
No full textThe finite-volume microscopic behaviour of a system in equilibrium is de¬termined by the Boltzmann-Gibbs prescription for probability measures in which probabilities are proportional to the exponential of some Hamiltonian, which describes the energy fluctuations while the total number of particles is fixed. For seeing a phase transition mathematically one has to investigate infinite-volume measures. The natural objects for describing dependences (due to e.g. a spin interaction) are conditional probabilities. These are com¬patible with a possibly non-unique measure on an infinite-volume space. The conditioned measures describe the behaviour of a finite set of spins while all others are fixed. This approach was proposed by Dobrushin, Lanford and Ruelle and is since referred to as the DLR approach. Kozlov and Sullivan gave an equivalent characterization of a Gibbs measure. A measure is Gibbs is it is uniformly nonnull and quasilocal. This property is very important in describing physical reality, since one is interested in modeling local systems which are not influenced by events at infinity. A fortiori a measure is not Gibbs, if one finds at least one discontinuity point (in the product topology) of the conditional probabilities w.r.t. the conditioning. Non-Gibbsianness can appear after applying different kind of transfor¬mations on Gibbs measures, such as renormalization group transformations, projections or time-evolution. In this thesis we study Gibbs and non-Gibbs aspects of continuous spin models during time-evolution. We obtain the ma¬jor part of the results for planar rotors which are spins moving on circles, these are stated in Chapters 3-5 and 7. Additionally we prove short-time Gibbsianness for unbounded spins in Chapter 6. In particular we will prove in Chapter 3, first that for every initial and dynamical inverse temperature the measure stays Gibbsian for short times. Secondly, for small inverse initial and dynamical temperatures the measure is Gibbs for all times. The third result states loss of Gibbsianness in a time interval for a planar rotor model without a field. Furthermore in Chapter 4 we prove loss and recovery of Gibbsianness for models in a field. For both cases where Gibbsianness was lost we assumed a low-temperature initial Gibbs measure and infinite-temperature dynamics (the initial measure converges towards an infinite temperature Gibbs mea¬sure). Analogously as in the Ising case, the presence of an initial external field is responsible for the measure to become Gibbs in finite time again. The following Chapter 5 deals with a planar rotor model in an alternat¬ing field in equilibrium, exhibiting a phase transition at low temperatures. Moreover it contains an argument how a similar proof can be used to prove phase coexistence for a planar rotor model in a field which consist of certain periodic or random configurations on crosses. In Chapter 6 we generalize the short-time conservation result for possibly non-Markovian dynamics for unbounded spins. Finally the last Chapter 7, which stands a bit apart, contains the first ex¬ample of Chaotic Temperature Dependence for compact spins. We construct a potential for a planar rotor system, thus a system of compact spins such that no ground state measure can be obtained by lowering the temperature. Thus the model depends chaotically on the temperature
Laudatio for Michael Aizenman
No full textEens per drie jaar reikt het Wiskundig Genootschap in opdracht van de Koninklijke Nederlandse Academie van Wetenschappen de Brouwermedaille uit aan een internationaal
toonaangevend onderzoeker. Hij wordt uitgenodigd om een voordracht te geven op het
Nederlands Mathematisch Congres, waar na afloop de laureaat de Brouwermedaille wordt
uitgereikt. In 2002 werd de medaille toegekend aan Michael Aizenman voor zijn bijdrage
aan de mathematische fysica. Op het Nederlands Mathematisch Congres in Eindhoven
op 4 april 2002 sprak Frank den Hollander de laudatio uit
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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