613 research outputs found
Evolution and stability of a magnetic vortex in small cylindrical ferromagnetic particle under applied field
The energy of a displaced magnetic vortex in a cylindrical particle made of
isotropic ferromagnetic material (magnetic dot) is calculated taking into
account the magnetic dipolar and the exchange interactions. Under the
simplifying assumption of small dot thickness the closed-form expressions for
the dot energy is written in a non-perturbative way as a function of the
coordinate of the vortex center. Then, the process of losing the stability of
the vortex under the influence of the externally applied magnetic field is
considered. The field destabilizing the vortex as well as the field when the
vortex energy is equal to the energy of a uniformly magnetized state are
calculated and presented as a function of dot geometry. The results (containing
no adjustable parameters) are compared to the recent experiment and are in good
agreement.Comment: 4 pages, 2 figures, RevTe
Homology and K--Theory Methods for Classes of Branes Wrapping Nontrivial Cycles
We apply some methods of homology and K-theory to special classes of branes
wrapping homologically nontrivial cycles. We treat the classification of
four-geometries in terms of compact stabilizers (by analogy with Thurston's
classification of three-geometries) and derive the K-amenability of Lie groups
associated with locally symmetric spaces listed in this case. More complicated
examples of T-duality and topology change from fluxes are also considered. We
analyse D-branes and fluxes in type II string theory on with torsion flux and demonstrate in details
the conjectured T-duality to with no flux. In the
simple case of , T-dualizing the circles reduces to
duality between with
flux and with no flux.Comment: 27 pages, tex file, no figure
Tightness of slip-linked polymer chains
We study the interplay between entropy and topological constraints for a
polymer chain in which sliding rings (slip-links) enforce pair contacts between
monomers. These slip-links divide a closed ring polymer into a number of
sub-loops which can exchange length between each other. In the ideal chain
limit, we find the joint probability density function for the sizes of segments
within such a slip-linked polymer chain (paraknot). A particular segment is
tight (small in size) or loose (of the order of the overall size of the
paraknot) depending on both the number of slip-links it incorporates and its
competition with other segments. When self-avoiding interactions are included,
scaling arguments can be used to predict the statistics of segment sizes for
certain paraknot configurations.Comment: 10 pages, 6 figures, REVTeX
Workflows for Quantitative Data Analysis in The Social Sciences
The background is given to how statistical analysis is used by quantitative social scientists. Developing statistical analyses requires substantial effort, yet there are important limitations in current practice. This has motivated the authors to create a more systematic and effective methodology with supporting tools. The approach to modelling quantitative data analysis in the social sciences is presented. Analysis scripts are treated abstractly as mathematical functions and concretely as web services. This allows individual scripts to be combined into high-level workflows. A comprehensive set of tools allows workflows to be defined, automatically validated and verified, and automatically implemented. The workflows expose opportunities for parallel execution, can define support for proper fault handling, and can be realised by non-technical users. Services, workflows and datasets can also be readily shared. The approach is illustrated with a realistic case study that analyses occupational position in relation to health
The Pivotal Role of Causality in Local Quantum Physics
In this article an attempt is made to present very recent conceptual and
computational developments in QFT as new manifestations of old and well
establihed physical principles. The vehicle for converting the
quantum-algebraic aspects of local quantum physics into more classical
geometric structures is the modular theory of Tomita. As the above named
laureate to whom I have dedicated has shown together with his collaborator for
the first time in sufficient generality, its use in physics goes through
Einstein causality. This line of research recently gained momentum when it was
realized that it is not only of structural and conceptual innovative power (see
section 4), but also promises to be a new computational road into
nonperturbative QFT (section 5) which, picturesquely speaking, enters the
subject on the extreme opposite (noncommutative) side.Comment: This is a updated version which has been submitted to Journal of
Physics A, tcilatex 62 pages. Adress: Institut fuer Theoretische Physik
FU-Berlin, Arnimallee 14, 14195 Berlin presently CBPF, Rua Dr. Xavier Sigaud
150, 22290-180 Rio de Janeiro, Brazi
The emerging structure of the Extended Evolutionary Synthesis: where does Evo-Devo fit in?
The Extended Evolutionary Synthesis (EES) debate is gaining ground in contemporary evolutionary biology. In parallel, a number of philosophical standpoints have emerged in an attempt to clarify what exactly is represented by the EES. For Massimo Pigliucci, we are in the wake of the newest instantiation of a persisting Kuhnian paradigm; in contrast, Telmo Pievani has contended that the transition to an EES could be best represented as a progressive reformation of a prior Lakatosian scientific research program, with the extension of its Neo-Darwinian core and the addition of a brand-new protective belt of assumptions and auxiliary hypotheses. Here, we argue that those philosophical vantage points are not the only ways to interpret what current proposals to ‘extend’ the Modern Synthesis-derived ‘standard evolutionary theory’ (SET) entail in terms of theoretical change in evolutionary biology. We specifically propose the image of the emergent EES as a vast network of models and interweaved representations that, instantiated in diverse practices, are connected and related in multiple ways. Under that assumption, the EES could be articulated around a paraconsistent network of evolutionary theories (including some elements of the SET), as well as models, practices and representation systems of contemporary evolutionary biology, with edges and nodes that change their position and centrality as a consequence of the co-construction and stabilization of facts and historical discussions revolving around the epistemic goals of this area of the life sciences. We then critically examine the purported structure of the EES—published by Laland and collaborators in 2015—in light of our own network-based proposal. Finally, we consider which epistemic units of Evo-Devo are present or still missing from the EES, in preparation for further analyses of the topic of explanatory integration in this conceptual framework
Chronic non-specific low back pain - sub-groups or a single mechanism?
Copyright 2008 Wand and O'Connell; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Background: Low back pain is a substantial health problem and has subsequently attracted a
considerable amount of research. Clinical trials evaluating the efficacy of a variety of interventions
for chronic non-specific low back pain indicate limited effectiveness for most commonly applied
interventions and approaches.
Discussion: Many clinicians challenge the results of clinical trials as they feel that this lack of
effectiveness is at odds with their clinical experience of managing patients with back pain. A
common explanation for this discrepancy is the perceived heterogeneity of patients with chronic
non-specific low back pain. It is felt that the effects of treatment may be diluted by the application
of a single intervention to a complex, heterogeneous group with diverse treatment needs. This
argument presupposes that current treatment is effective when applied to the correct patient.
An alternative perspective is that the clinical trials are correct and current treatments have limited
efficacy. Preoccupation with sub-grouping may stifle engagement with this view and it is important
that the sub-grouping paradigm is closely examined. This paper argues that there are numerous
problems with the sub-grouping approach and that it may not be an important reason for the
disappointing results of clinical trials. We propose instead that current treatment may be ineffective
because it has been misdirected. Recent evidence that demonstrates changes within the brain in
chronic low back pain sufferers raises the possibility that persistent back pain may be a problem of
cortical reorganisation and degeneration. This perspective offers interesting insights into the
chronic low back pain experience and suggests alternative models of intervention.
Summary: The disappointing results of clinical research are commonly explained by the failure of
researchers to adequately attend to sub-grouping of the chronic non-specific low back pain
population. Alternatively, current approaches may be ineffective and clinicians and researchers may
need to radically rethink the nature of the problem and how it should best be managed
Accurate and Rapid Estimation of Phosphene Thresholds (REPT)
To calibrate the intensity of transcranial magnetic stimulation (TMS) at the occipital pole, the phosphene threshold is used as a measure of cortical excitability. The phosphene threshold (PT) refers to the intensity of magnetic stimulation that induces illusory flashes of light (phosphenes) on a proportion of trials. The existing PT estimation procedures lack the accuracy and mathematical rigour of modern threshold estimation methods. We present an improved and automatic procedure for estimating the PT which is based on the well-established Ψ Bayesian adaptive staircase approach. To validate the new procedure, we compared it with another commonly used procedure for estimating the PT. We found that our procedure is more accurate, reliable, and rapid when compared with an existing PT measurement procedure. The new procedure is implemented in Matlab and works automatically with the Magstim Rapid2 stimulator using a convenient graphical user interface. The Matlab program is freely available for download
An Introduction to Conformal Field Theory
A comprehensive introduction to two-dimensional conformal field theory is
given.Comment: 69 pages, LaTeX; references adde
Decomposition of operator semigroups on W*-algebras
We consider semigroups of operators on a W-algebra and prove, under
appropriate assumptions, the existence of a Jacobs-DeLeeuw-Glicksberg type
decomposition. This decomposition splits the algebra into a "stable" and
"reversible" part with respect to the semigroup and yields, among others, a
structural approach to the Perron-Frobenius spectral theory for completely
positive operators on W-algebras.Comment: referee's comments incorporated. To appear in Semigroup Foru
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