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Search Results: 1 - 10 of 3291 matches for " Anders Karlsson "
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On the Efficiency and Gain of Antennas
Anders Karlsson
PIER , 2013, DOI: 10.2528/PIER12110504
Abstract: The fundamental limits of the gain and efficiency of an antenna are explored. These are very important quantities for e.g., superdirective arrays. The antenna is in this paper confined in a sphere and all of the currents are assumed to run in a material with a given conductivity. It is shown that one can find the current distribution in the sphere that optimizes the gain, given the frequency and the radius of the sphere. The results indicate the distribution of antenna elements in an antenna array in order to maximize gain, or efficiency. The analysis is based on the expansion of the electromagnetic fields in terms of vector spherical harmonics. Explicit expressions for the limits of gain and efficiency, and the corresponding current densities, are derived for different types of antennas.
Two extensions of Thurston's spectral theorem for surface diffeomorphisms
Anders Karlsson
Mathematics , 2012,
Abstract: Thurston obtained a classification of individual surface homeomorphisms via the dynamics of the corresponding mapping class elements on Teichm\"uller space. In this paper we present certain extended versions of this, first, to random products of homeomorphisms and second, to holomorphic self-maps of Teichm\"uller spaces.
Dynamics of Hilbert nonexpansive maps
Anders Karlsson
Mathematics , 2013,
Abstract: In his work on the foundations of geometry, Hilbert observed that a formula which appeared in works by Beltrami, Cayley, and Klein, gives rise to a complete metric on any bounded convex domain. Some decades later, Garrett Birkhoff and Hans Samelson noted that this metric has interesting applications, when considering certain maps of convex cones that contract the metric. Such situations have since arisen in many contexts, pure and applied, and could be called nonlinear Perron-Frobenius theory. This note centers around one dynamical aspect of this theory.
On the dynamics of isometries
Anders Karlsson
Mathematics , 2005, DOI: 10.2140/gt.2005.9.2359
Abstract: We provide an analysis of the dynamics of isometries and semicontractions of metric spaces. Certain subsets of the boundary at infinity play a fundamental role and are identified completely for the standard boundaries of CAT(0)-spaces, Gromov hyperbolic spaces, Hilbert geometries, certain pseudoconvex domains, and partially for Thurston's boundary of Teichmueller spaces. We present several rather general results concerning groups of isometries, as well as the proof of other more specific new theorems, for example concerning the existence of free nonabelian subgroups in CAT(0)-geometry, iteration of holomorphic maps, a metric Furstenberg lemma, random walks on groups, noncompactness of automorphism groups of convex cones, and boundary behaviour of Kobayashi's metric.
Articular cartilage stem cell signalling
Camilla Karlsson, Anders Lindahl
Arthritis Research & Therapy , 2009, DOI: 10.1186/ar2753
Abstract: Articular cartilage has been considered a post-mitotic tissue with virtually no cellular turnover. This has been based on the fact that the tissue is hypocellular and avascular and relies on diffusion for its nutrient supply. In the previous issue of Arthritis Research & Therapy, Grogan and colleagues [1] addressed the question of the localization of progenitor cells in healthy and osteoarthritic (OA) cartilage using Notch-1, Stro-1, and vascular cell adhesion molecule-1 (VCAM-1) as markers for stem cells.Articular cartilage has been proposed to consist of only terminally differentiated cells in adults, lacking progenitor cells – a dogma well-established in the textbooks. However, this dogma has been challenged in recent years [2-4] by the hypothesis that a progenitor cell population resides in the superficial zone of the cartilage. An additional challenge to the dogma is the fact that articular cartilage is not homogenous; instead, biochemical and morphological variations are seen from the surface zone (SZ) through the middle zone (MZ) and down to the deep zone (DZ). In the SZ, cells are flattened and secrete lubricin [5]; in the MZ, the cells are rounded and arranged in columnar structure and produce cartilage intermediate layer protein (CILP) [6]; but in the DZ, the cells are considerably larger and express type X collagen and alkaline phosphatase.Grogan and colleagues [1] related their finding to the three different zones in hyaline cartilage. The authors demonstrated similar staining patterns for the three makers but with a distinct zonal distribution pattern in healthy cartilage. The highest frequencies of stained cells were found in the SZ.The presence of progenitor cells is a key component to rapid and successful regeneration of a variety of tissues. The few studies performed concerning the regenerative potential of embryonic cartilage are somewhat conflicting. Namba and colleagues [7] reported that laceration of foetal cartilage has an intrinsic reparative
Evidence-based practice - anything goes?
Patrik Karlsson, Anders Bergmark
Nordic Studies on Alcohol and Drugs , 2012, DOI: 10.2478/v10199-012-0022-y
Broadcasting of entanglement at a distance using linear optics and telecloning of entanglement
Iulia Ghiu,Anders Karlsson
Physics , 2005, DOI: 10.1103/PhysRevA.72.032331
Abstract: We propose a scheme for broadcasting entanglement at a distance based on linear optics. We show that an initial polarization entangled state can be simultaneously split and transmitted to a pair of observers situated at different locations with the help of two conditional Bell-state analyzers based on two beam splitters characterized by the same reflectivity R. In particular for R=1/3 the final states coincide with the output states obtained by the broadcasting protocol proposed by Buzek et al. [Phys. Rev. A 55, 3327 (1997)]. Further we present a different protocol called telecloning of entanglement, which combines the many-to-many teleportation and nonlocal optimal asymmetric cloning of an arbitrary entangled state. This scheme allows the optimal transmission of the two nonlocal optimal clones of an entangled state to two pairs of spatially separated receivers.
An explicit kernel-split panel-based Nystr?m scheme for integral equations on axially symmetric surfaces
Johan Helsing,Anders Karlsson
Physics , 2013,
Abstract: A high-order accurate, explicit kernel-split, panel-based, Fourier-Nystr\"om discretization scheme is developed for integral equations associated with the Helmholtz equation in axially symmetric domains. Extensive incorporation of analytic information about singular integral kernels and on-the-fly computation of nearly singular quadrature rules allow for very high achievable accuracy, also in the evaluation of fields close to the boundary of the computational domain.
Determination of normalized magnetic eigenfields in microwave cavities
Johan Helsing,Anders Karlsson
Physics , 2014,
Abstract: The magnetic field integral equation for axially symmetric cavities with perfectly conducting surfaces is discretized according to a high-order convergent Fourier--Nystr\"om scheme. The resulting solver is used to determine eigenwavenumbers and normalized magnetic eigenfields to very high accuracy in the entire computational domain.
Determination of normalized electric eigenfields in microwave cavities with sharp edges
Johan Helsing,Anders Karlsson
Physics , 2015,
Abstract: The magnetic field integral equation for axially symmetric cavities with perfectly conducting piecewise smooth surfaces is discretized according to a high-order convergent Fourier--Nystr\"om scheme. The resulting solver is used to accurately determine eigenwavenumbers and normalized electric eigenfields in the entire computational domain.
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