130,404 research outputs found
Comment on "What does the Letelier-Gal'tsov metric describe?"
No full text3 pagesWe show that the Letelier-Gal'tsov (LG) metric describing multiple crossed strings in relative motion does solve the Einstein equations, in spite of the discontinuity uncovered recently by Krasnikov [gr-qc/0502090] provided the strings are straight and moving with constant velocities
Interpolating Black Holes In Dilaton-axion Gravity
No full textA 2p + 5 parametric family of black holes is constructed in dilaton-axion gravity with p vector fields using a holomorphic representation of U-duality in three dimensions. The metric of the non-extremal black holes has a Reissner-Nordström-type structure and generically possesses an internal horizon. However, in the extremal limit the generic solution exhibits a dilatonic-type null singularity. The solution thus interpolates between the well known Reissner-Nordström and charged dilatonic spacetimes. Only in the case of the orthogonal electric and magnetic charges (if p > 1) may the extremal solution have a non-singular event horizon.141L9L14Gibbons, G.W., (1982) Nucl. Phys. B, 207, p. 337Gibbons, G.W., Maeda, K., (1988) Nucl. Phys. B, 298, p. 741Garfinkle, D., Horowitz, G.T., Strominger, A., (1991) Phys. Rev. D, 43, p. 3140Garfinkle, D., Horowitz, G.T., Strominger, A., (1992) Phys. Rev. D, 45, p. 3888. , erratumKallosh, R., Linde, A., Ortin, T., Peet, A., Van Proyen, A., (1992) Phys. Rev. D, 46, p. 5278Bergshoeff, E., Kallosh, R., Ortin, T., Stationary Axion/Dilaton Solutions and Supersymmetry, , Preprint UG-3/96 SU-ITP-19 CERN-TH/96-106 Hep-th/9605059Gal'tsov, D.V., Kechkin, O.V., (1994) Phys. Rev. D, 50, p. 7394Gal'tsov, D.V., (1995) Phys. Rev. Lett., 74, p. 2863Gal'tsov, D.V., Kechkin, O.V., (1995) Phys. Lett., 361 B, p. 52Gal'tsov, D.V., Square of general relativity (1996) A Talk at the First Australasian Conf. on General Relativity and Gravitation (Adelaide), , DTP-MSU 96/14 Gr-qc/9608021 to be publishedGibbons, G., Kallosh, R., Kol, B., Moduli, Scalar Charges, and the First Law of Black Hole Thermodynamics, , Preprint SU-ITP-96-35 Hep-th/9607108Clément, G., Gal'tsov, D., Stationary BPS Solutions to Dilaton-axion Gravity, , Preprint GCR-96/07/02 DTP-MSU/96-11 Hep-th/9607043Cvetič, M., Youm, D., General Static Spherically Symmetric Black Holes of Heterotic String on a Six Torus, , Preprint IASSNS-HEP-95-107 Hep-th/9512127Cvetič, M., Tseytlin, A., (1996) Phys. Rev. D, 53, p. 5619Cvetič, M., Tseytlin, A., Non-extreme Black Holes from Non-extreme Intersecting M-branes, , Preprint DAMTP/R/96/27 Imperial/TP/95-96/52 Hep-th/9606033Cvetič, M., Hull, C.M., Black Holes and U-duality, , Preprint DAMTP/R/96/31 QMW-96-12 Hep-th/960619
MeSH term explosion and author rank improve expert recommendations
No full textInformation overload is an often-cited phenomenon that reduces the productivity, efficiency and efficacy of scientists. One challenge for scientists is to find appropriate collaborators in their research. The literature describes various solutions to the problem of expertise location, but most current approaches do not appear to be very suitable for expert recommendations in biomedical research. In this study, we present the development and initial evaluation of a vector space model-based algorithm to calculate researcher similarity using four inputs: 1) MeSH terms of publications; 2) MeSH terms and author rank; 3) exploded MeSH terms; and 4) exploded MeSH terms and author rank. We developed and evaluated the algorithm using a data set of 17,525 authors and their 22,542 papers. On average, our algorithms correctly predicted 2.5 of the top 5/10 coauthors of individual scientists. Exploded MeSH and author rank outperformed all other algorithms in accuracy, followed closely by MeSH and author rank. Our results show that the accuracy of MeSH term-based matching can be enhanced with other metadata such as author rank
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
No full textBoth spending and tax policies have been implemented in the United States with the goal of stimulating private sector research and development (R&D). Karier questions whether current R&D policy, especially the research and experimentation tax credit, can contribute to closing the gap between nondefense expenditures on R&D in the United States and such expenditures in other countries, such as Japan and Germany. He also explores possible changes to our current R&D policy to make it more effective.
Completing characterization of photon orbits in Kerr and Kerr-Newman metrics
No full text© 2019 American Physical Society. Recently, several new characteristics have been introduced to describe null geodesic structure of stationary spacetimes, such as photon regions (PR) and transversely trapping surfaces (TTS). The former are three-dimensional domains confining the spherical photon orbits, while the latter are closed two-surface of spherical topology which fill the other regions, called transversely trapping region (TTR). It is argued that in a generic stationary axisymmetric spacetime it is natural to consider also the nonclosed TTSs of the geometry of spherical cups, satisfying the same conditions locally ("partial" TTS or PTTS), which fill the three-dimensional regions, PTTR. We then show that PR, TTR and PTTR together with the corresponding antitrapping regions constitute the complete set of domains filling the entire Kerr-like spacetimes. This construction provides a novel optical description of such spacetimes which does not require explicit solution of geodesic equations. Applying this analysis to Kerr-Newman metrics (including the overspinning ones) we reveal four different optical types for different sets of the rotation and charge parameters. To illustrate their properties we extend the Synge and Semerak analysis of photon escape to stationary spacetimes and construct density graphs describing escape of photons from all the above regions
Photon trapping in static axially symmetric spacetime
No full text© 2019 American Physical Society. Recently, several new characteristics have been introduced to describe null geodesic structure of strong gravitational fields, such as photon regions, transversely trapping surfaces, and some of their generalizations. They provide an alternative and concise way of describing the lensing and shadow features of compact objects without recurring to the complete integration of geodesic equations. Here we test this construction in the case of the static asymptotically flat Weyl metrics where the geodesic equations are nonseparable and cannot be integrated analytically, while the characteristic surfaces can be described in a closed form. Specifically, we consider the gamma-metrics which attracted attention as an alternative to the standard picture of a black hole. We further develop our formalism by introducing a more detailed specification of transversely trapping surfaces in terms of their principal curvatures. Surprisingly, we find certain structures, such as photon regions, previously known only in stationary spacetimes, in the static case without spherical symmetry too. These photon regions can be viewed as photon spheres, "thickened" by oblateness of the metric
Oxidation of D=3 cosets and Bonnor dualities in
No full textInternational audienceBonnor's map in General Relativity is duality between (dimensionally reduced) vacuum gravity and static truncation of electro-vacuum theory. It was used as a tool to generate an exact solution of electro-vacuum from some vacuum solution. It can be expected that similar dualities will be useful for solution generation in higher-dimensional theories too. Here we study such maps within a class of theories in dimensions using oxidation of D=3 cosets and consistent truncation of the corresponding theories. Our class includes those theories whose D=3 symmetries are subgroups of . It contains six-dimensional minimal supergravity, five-dimensional minimal and supergravities and a number of four-dimensional theories which attracted attention recently in the search of exact solutions. We give explicit reduction/truncation formulas relating different theories in dimensions in terms of metrics and matter fields and discuss various alternative duality chains between them
Dispelling the Myths Behind First-author Citation Counts
No full textWe 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
- …
