1,721,049 research outputs found
A fast converging separable expansion for realistic nuclear potentials
No full textA new separable-expansion method for realistic multichannel scattering problems is proposed. The approximate finite-rank potentials are obtained by resorting to a complete set of sturmians referring to a fixed negative energy. These basis functions are solutions of a coupled-channel auxiliary eigenvalue problem embodying the main physical features of the original interactions. The coupled sturmian equations are solved through a simple diagonalization procedure by employing another set of auxiliary sturmians given in closed analytic form. The method is applied to uncoupled as well as to coupled states for the Reid potential. Good convergence properties are found both on and off the energy shell. The method can be applied to general coupled-channel problems, and can be extended so as to treat absorptive and/or energy-dependent interaction
Re-examination of the πN N N-N N N problem
No full textWe discuss the dynamical equations for the πN N N-N N N} system. We introduce the Afnan-Blankleider-Avishai-Mizutani equations in a very direct and immediate way, using the diagrammatic method and special properties of the four-body transition operators. The connection between this four-body unitary theory and a more phenomenological model for the3He + π→N N Nprocess, recently developed by the authors, is established
Analysis of the optical potential with coupled-channel scattering equations: Energy dependence and coordinate-space behavior
No full textWe apply a Sturmian-expansion method we have recently developed to perform an exact multichannel analysis of the optical potential. By combining this method with momentum-space integral-equation techniques we can obtain a finite-rank representation of the nonlocal optical potential in which coupling effects are exactly taken into account. In the framework of this approach we provide also detailed expressions of the various contributions arising when couplings are treated perturbatively. These iterative-perturbative approximation schemes are generally employed in phenomenological optical-potential calculations. Our results do not apply to low-order terms only, but can be extended to any desired order. We find that a proper treatment of inelastic-inelastic couplings may be necessary in order to reproduce the detailed nonlocal structure of the optical potential. We then perform an energy-dependent analysis of the resulting optical potential. To this end, the rapid fluctuations in energy due to compound or quasicompound resonances are subtracted off by means of a direct algebraic procedure. The connection of this subtraction method with Feshbachs general reaction theory is clarified. We analyze the energy dependence of the smooth optical interaction in the light of a dispersion-theoretic approach, by resorting to simple analytic parametrizations of the real and imaginary parts. Both a model multichannel problem and the realistic neutron-208Pb case have been considered
Separable expansions for realistic multichannel scattering problems
No full textA new approach to the multichannel scattering problem with realistic local or nonlocal interactions is developed. By employing the negative-energy solutions of uncoupled Sturmian eigenvalue problems referring to simple auxiliary potentials, the coupling interactions appearing in the original multichannel problem are approximated by finite-rank potentials. By resorting to integral-equation techniques the coupled-channel equations are then reduced to linear algebraic equations which can be straightforwardly solved. Compact algebraic expressions for the relevant scattering matrix elements are thus obtained. The convergence of the method is tested in the single-channel case with realistic optical potentials. Excellent agreement is obtained with a few terms in the separable expansion for both real and absorptive interactions
On the coupled-channel calculation of the optical potential with full inclusion of coupling effects
No full textWe develop a method for a coupled-channel calculation of the optical potential in which the multichannel Green's function is evaluated exactly. The quantum-mechanical coupled-channel problem is solved by expanding the multichannel interactions in terms of coupled sturmians. Closed- and open-channel effects are straightforwardly included by the use of energy-dependent basis functions. An application in the framework of a schematic multichannel model is developed
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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