1,720,964 research outputs found
Boundary Triples for the Dirac Operator with Coulomb-Type Spherically Symmetric Perturbations
Full text linkWe determine explicitly a boundary triple for the Dirac operator in , for and , with . Consequently we determine all the self-adjoint realizations of in terms of the behaviour of the functions of their domain in the origin. When , we discuss the problem of selecting the distinguished extension requiring that its domain is included in the domain of the appropriate quadratic form
Self-Adjoint Extensions for the Dirac Operator with Coulomb-Type Spherically Symmetric Potentials
Full text linkWe describe the self-adjoint realizations of the operator , for , and , for . We characterize the self-adjointness in terms of the behaviour of the functions of the domain in the origin, exploiting Hardy-type estimates and trace lemmas. Finally, we describe the distinguished extension.Istituto Italiano di Alta Matematica "F. Severi
A Hardy-type inequality and some spectral characterizations for the Dirac–Coulomb operator
Full text linkWe prove a sharp Hardy-type inequality for the Dirac operator. We exploit this inequality to obtain spectral properties of the Dirac operator perturbed with Hermitian matrix-valued potentials of Coulomb type: we characterise its eigenvalues in terms of the Birman–Schwinger principle and we bound its discrete spectrum from below, showing that the ground-state energy is reached if and only if verifies some rigidity conditions. In the particular case of an electrostatic potential, these imply that is the Coulomb potential
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
Singular Perturbation of the Dirac Hamiltonian
Full text linkThis thesis is devoted to the study of the Dirac Hamiltonian perturbed by delta-type potentials and Coulomb-type potentials.
We analysed the delta-shell interaction on bounded and smooth domains and its approximation by the coupling of the free Dirac operator with shrinking short-range potentials. Under certain hypothesis of smallness of a regular potential, in three dimensions, we prove that the Dirac operator coupled with a suitable rescaling of the potential converges to the Hamiltonian coupled with the delta-shell potential, in the strong resolvent sense. Nevertheless, the coupling constant depends non-linearly on the potential: Klein's Paradox comes into play.
As a particular case, we pay to focus on the Dirac Operator with spherical delta-shell interactions. We characterise the eigenstates of the couplings by finding sharp constants and minimisers of some precise inequalities related to an uncertainty principle. We also explore the spectral relation between the shell interaction and its approximation by short-range potentials with shrinking support, improving previous results.
Finally, we investigate the Dirac operator perturbed by a particular class of Coulomb-type spherically symmetric potentials. We describe the self-adjoint realisations of this operator in terms of the behaviour of the functions of the domain in the origin, and we provide Hardy-type estimates on them. At the end, we describe the distinguished extension
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
- …
