83,128 research outputs found
Covariant Quantum Fields on Noncommutative Spacetimes
A spinless covariant field on Minkowski spacetime \M^{d+1} obeys the
relation where
is an element of the Poincar\'e group \Pg and is its unitary representation on quantum vector states. It
expresses the fact that Poincar\'e transformations are being unitary
implemented. It has a classical analogy where field covariance shows that
Poincar\'e transformations are canonically implemented. Covariance is
self-reproducing: products of covariant fields are covariant. We recall these
properties and use them to formulate the notion of covariant quantum fields on
noncommutative spacetimes. In this way all our earlier results on dressing,
statistics, etc. for Moyal spacetimes are derived transparently. For the Voros
algebra, covariance and the *-operation are in conflict so that there are no
covariant Voros fields compatible with *, a result we found earlier. The notion
of Drinfel'd twist underlying much of the preceding discussion is extended to
discrete abelian and nonabelian groups such as the mapping class groups of
topological geons. For twists involving nonabelian groups the emergent
spacetimes are nonassociative.Comment: 20 page
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Insights into Activation Mechanisms of Store-Operated TRPC1 Channels in Vascular Smooth Muscle.
In vascular smooth muscle cells (VMSCs), the stimulation of store-operated channels (SOCs) mediate Ca2+ influx pathways which regulate important cellular functions including contraction, proliferation, migration, and growth that are associated with the development of vascular diseases. It is therefore important that we understand the biophysical, molecular composition, activation pathways, and physiological significance of SOCs in VSMCs as these maybe future therapeutic targets for conditions such as hypertension and atherosclerosis. Archetypal SOCs called calcium release-activated channels (CRACs) are composed of Orai1 proteins and are stimulated by the endo/sarcoplasmic reticulum Ca2+ sensor stromal interaction molecule 1 (STIM1) following store depletion. In contrast, this review focuses on proposals that canonical transient receptor potential (TRPC) channels composed of a heteromeric TRPC1/C5 molecular template, with TRPC1 conferring activation by store depletion, mediate SOCs in native contractile VSMCs. In particular, it summarizes our recent findings which describe a novel activation pathway of these TRPC1-based SOCs, in which protein kinase C (PKC)-dependent TRPC1 phosphorylation and phosphatidylinositol 4,5-bisphosphate (PIP2) are obligatory for channel opening. This PKC- and PIP2-mediated gating mechanism is regulated by the PIP2-binding protein myristoylated alanine-rich C kinase (MARCKS) and is coupled to store depletion by TRPC1-STIM1 interactions which induce Gq/PLCÎČ1 activity. Interestingly, the biophysical properties and activation mechanisms of TRPC1-based SOCs in native contractile VSMCs are unlikely to involve Orai1
Numerical and experimental assessment of the modal curvature method for damage detection in plate structures
This paper is concerned with the use of numerically obtained modal curvatures for damage detection in both isotropic and composite laminated plates. Numerical simulations are carried out by using COMSOL Multiphysics as FEM solver of the governing equations, in which a Mindlin-Reissner plate model is assumed and defects are introduced as localized smoothed variations of the baseline (healthy) configuration. Experiments are also performed on steel and aluminum plates using scanning laser vibrometry. This study confirms that the central difference method greatly amplifies the measurement errors and its application leads to ineffective predictions for damage detection, even after denoising. As a consequence, different numerical techniques should be explored to allow the use of numerically obtained modal curvatures for structural health monitoring. Herein, the Savitzky-Golay filter (or least-square smoothing filter) is considered for the numerical differentiation of noisy data
Numerical and experimental assessment of the modal curvature method for damage detection in plate structures
Use of modal curvatures obtained from modal displacement data for damage detection in isotropic and composite laminated plates is addressed through numerical examples and experimental tests. Numerical simulations are carried out employing COMSOL Multiphysics as finite element solver of the equations governing the Mindlin-Reissner plate model. Damages are introduced as localized non-smooth variations of the bending stiffness of the baseline (healthy) configuration. Experiments are also performed on steel and aluminum plates using scanning laser vibrometry. The obtained results confirm that use of the central difference method to compute modal curvatures greatly amplifies the measurement errors and its application leads to unreliable predictions for damage detection, even after denoising. Therefore, specialized ad hoc numerical techniques must be suitably implemented to enable structural health monitoring via modal curvature changes. In this study, the Savitzky-Golay filter (also referred to as least-square smoothing filter) is considered for the numerical differentiation of noisy data. Numerical and experimental results show that this filter is effective for the reliable computation of modal curvature changes in plate structures due to defects and/or damages
Multi-waveband Emission Maps of Blazars
We are leading a comprehensive multi-waveband monitoring program of 34
gamma-ray bright blazars designed to locate the emission regions of blazars
from radio to gamma-ray frequencies. The "maps" are anchored by sequences of
images in both total and polarized intensity obtained with the VLBA at an
angular resolution of ~ 0.1 milliarcseconds. The time-variable linear
polarization at radio to optical wavelengths and radio to gamma-ray light
curves allow us to specify the locations of flares relative to bright
stationary features seen in the images and to infer the geometry of the
magnetic field in different regions of the jet. Our data reveal that some
flares occur simultaneously at different wavebands and others are only seen at
some of the frequencies. The flares are often triggered by a superluminal knot
passing through the stationary "core" on the VLBA images. Other flares occur
upstream or even parsecs downstream of the core.Comment: 5 pages, including 2 figures; to be published in Journal of
Astrophysics and Astronomy, as part of proceedings of the meeting
"Multiwavelength Variability of Blazars" held in Guangzhou, China, in
September 201
Random planar graphs and the London street network
In this paper we analyse the street network of London both in its primary and dual representation. To understand its properties, we consider three idealised models based on a grid, a static random planar graph and a growing random planar graph. Comparing the models and the street network, we find that the streets of London form a self-organising system whose growth is characterised by a strict interaction between the metrical and informational space. In particular, a principle of least effort appears to create a balance between the physical and the mental effort required to navigate the city
IEEE 802.11n MAC frame aggregation mechanisms for next-generation high-throughput WLANs [Medium access control protocols for wireless LANs]
IEEE 802.11n is an ongoing next-generation wireless LAN standard that supports a very highspeed connection with more than 100 Mb/s data throughput measured at the medium access control layer. This article investigates the key MAC enhancements that help 802.11n achieve high throughput and high efficiency. A detailed description is given for various frame aggregation mechanisms proposed in the latest 802.11n draft standard. Our simulation results confirm that A-MSDU, A-MPDU, and a combination of these methods improve extensively the channel efficiency and data throughput. We analyze the performance of each frame aggregation scheme in distinct scenarios, and we conclude that overall, the two-level aggregation is the most efficacious
REGULATORY APPROVAL OF NEW MEDICAL DEVICES: A CROSS SECTIONAL STUDY
Objective To investigate the regulatory approval of new medical devices. Design Cross sectional study of new medical devices reported in the biomedical literature. Data sources PubMed was searched between 1 January 2000 and 31 December 2004 to identify clinical studies of new medical devices. The search was carried out during this period to allow time for regulatory approval. Eligibility criteria for study selection Articles were included if they reported a clinical study of a new medical device and there was no evidence of a previous clinical study in the literature. We defined a medical device according to the US Food and Drug Administration as an âinstrument, apparatus, implement, machine, contrivance, implant, in vitro reagent, or other similar or related article.â Main outcome measures Type of device, target specialty, and involvement of academia or of industry for each clinical study. The FDA medical databases were then searched for clearance or approval relevant to the device. Results 5574 titles and abstracts were screened, 493 full text articles assessed for eligibility, and 218 clinical studies of new medical devices included. In all, 99/218 (45%) of the devices described in clinical studies ultimately received regulatory clearance or approval. These included 510(k) clearance for devices determined to be âsubstantially equivalentâ to another legally marketed device (78/99; 79%), premarket approval for high risk devices (17/99; 17%), and others (4/99; 4%). Of these, 43 devices (43/99; 43%) were actually cleared or approved before a clinical study was published. Conclusions We identified a multitude of new medical devices in clinical studies, almost half of which received regulatory clearance or approval. The 510(k) pathway was most commonly used, and clearance often preceded the first published clinical study
Non-Pauli Effects from Noncommutative Spacetimes
Noncommutative spacetimes lead to nonlocal quantum field theories (qft's)
where spin-statistics theorems cannot be proved. For this reason, and also
backed by detailed arguments, it has been suggested that they get corrected on
such spacetimes leading to small violations of the Pauli principle. In a recent
paper \cite{Pauli}, Pauli-forbidden transitions from spacetime noncommutativity
were calculated and confronted with experiments. Here we give details of the
computation missing from this paper. The latter was based on a spacetime
different from the Moyal plane. We argue that it
quantizes time in units of . Energy is then conserved only mod
. Issues related to superselection rules raised by non-Pauli
effects are also discussed in a preliminary manner.Comment: 15 Pages, 1 Table, Full details and further developments of
arXiv:1003.2250. This version is close to the one accepted by JHE
A Special Homotopy Continuation Method For A Class of Polynomial Systems
A special homotopy continuation method, as a combination of the polyhedral
homotopy and the linear product homotopy, is proposed for computing all the
isolated solutions to a special class of polynomial systems. The root number
bound of this method is between the total degree bound and the mixed volume
bound and can be easily computed. The new algorithm has been implemented as a
program called LPH using C++. Our experiments show its efficiency compared to
the polyhedral or other homotopies on such systems. As an application, the
algorithm can be used to find witness points on each connected component of a
real variety
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