A Galerkin boundary element method for high frequency scattering by convex polygons

In this paper we consider the problem of time-harmonic acoustic scattering in two dimensions by convex polygons. Standard boundary or finite element methods for acoustic scattering problems have a computational cost that grows at least linearly as a function of the frequency of the incident wave. Here we present a novel Galerkin boundary element method, which uses an approximation space consisting of the products of plane waves with piecewise polynomials supported on a graded mesh, with smaller elements closer to the corners of the polygon. We prove that the best approximation from the approximation space requires a number of degrees of freedom to achieve a prescribed level of accuracy that grows only logarithmically as a function of the frequency. Numerical results demonstrate the same logarithmic dependence on the frequency for the Galerkin method solution. Our boundary element method is a discretization of a well-known second kind combined-layer-potential integral equation. We provide a proof that this equation and its adjoint are well-posed and equivalent to the boundary value problem in a Sobolev space setting for general Lipschitz domains

Exterior optical cloaking and illusions by using active sources: a boundary element perspective

Recently, it was demonstrated that active sources can be used to cloak any objects that lie outside the cloaking devices [Phys. Rev. Lett. \textbf{103}, 073901 (2009)]. Here, we propose that active sources can create illusion effects, so that an object outside the cloaking device can be made to look like another object. invisibility is a special case in which the concealed object is transformed to a volume of air. From a boundary element perspective, we show that active sources can create a nearly "silent" domain which can conceal any objects inside and at the same time make the whole system look like an illusion of our choice outside a virtual boundary. The boundary element method gives the fields and field gradients (which can be related to monopoles and dipoles) on continuous curves which define the boundary of the active devices. Both the cloaking and illusion effects are confirmed by numerical simulations

A One Month Review of the Types of Medical Emergencies and their Treatment Outcomes at Two Urban Public Health Clinics.

INTRODUCTION Our study was to examine prevalence and treatment outcomes of medical emergencies at two urban public health clinics in the Petaling district, Selangor, Malaysia. METHODS A prospective universal sampling was employed to recruit all emergencies over one month period (12 April to 11 May 2011). A structured case record form was used to capture demographic data, whether the index case was selfpresenting or decided by health care workers as a medical emergency, presenting complaints, diagnoses, concurrent chronic diseases and their treatment outcomes at the clinic level. Emergency presentations and diagnoses were classified according to the International Classification of Primary Care, revised second edition (ICPC-2-R). RESULTS A total of 125 medical emergencies with 276 presenting complaints were recorded. The mean age was 30.7 years old (SD 19.9). The prevalence of medical emergency was 0.56% (125/22,320). Chief complaints were mainly from ICPC-2-R chapter R (respiratory system) and chapter A (general and unspecified), 40.0% and 28.0% respectively. The most common diagnosis was acute exacerbation of bronchial asthma (34.6%). Forty percent were referred to hospitals. After adjusting for age and gender, patients who presented with painful emergency (OR 4.9 95% CI 2.0 to 11.7), cardiovascular emergency (OR 63.4 95% CI 12.9 to 310.4) and non-respiratory emergency were predictors of hospital referral (OR 4.6 95% CI 1.1 to 19.1). CONCLUSION There was about one medical emergency for every 200 patients presenting to these urban public polyclinics which were mainly acute asthma. More than half were discharged well and given a follow-up

Balancing Minimum Spanning and Shortest Path Trees

This paper give a simple linear-time algorithm that, given a weighted digraph, finds a spanning tree that simultaneously approximates a shortest-path tree and a minimum spanning tree. The algorithm provides a continuous trade-off: given the two trees and epsilon > 0, the algorithm returns a spanning tree in which the distance between any vertex and the root of the shortest-path tree is at most 1+epsilon times the shortest-path distance, and yet the total weight of the tree is at most 1+2/epsilon times the weight of a minimum spanning tree. This is the best tradeoff possible. The paper also describes a fast parallel implementation.Comment: conference version: ACM-SIAM Symposium on Discrete Algorithms (1993

Slow light in photonic crystals

The problem of slowing down light by orders of magnitude has been extensively discussed in the literature. Such a possibility can be useful in a variety of optical and microwave applications. Many qualitatively different approaches have been explored. Here we discuss how this goal can be achieved in linear dispersive media, such as photonic crystals. The existence of slowly propagating electromagnetic waves in photonic crystals is quite obvious and well known. The main problem, though, has been how to convert the input radiation into the slow mode without loosing a significant portion of the incident light energy to absorption, reflection, etc. We show that the so-called frozen mode regime offers a unique solution to the above problem. Under the frozen mode regime, the incident light enters the photonic crystal with little reflection and, subsequently, is completely converted into the frozen mode with huge amplitude and almost zero group velocity. The linearity of the above effect allows to slow light regardless of its intensity. An additional advantage of photonic crystals over other methods of slowing down light is that photonic crystals can preserve both time and space coherence of the input electromagnetic wave.Comment: 96 pages, 12 figure

Environmental threats to the health of children: the Asian perspective.

Environmental Health Perspectives10810989-992EVHP

The role of attention in the perception of music structure

Existing models of the perception of musical structure mostly do not account for the fact that listeners’ hearings are known to vary substantially: the same passage can be interpreted differently by different listeners, or by the same listener at different times. Attention””the deliberate or unconscious focus a listener may place on a particular aspect of the music, such as its melody or rhythm””seems to play a role in the perception of structure, but whether it is an important cause of grouping preferences or the product of them is unclear. We study how paying attention to musical features (including harmony, melody, rhythm and timbre) influences grouping decisions. The experiments use composed musical stimuli exhibiting changes in particular features by design; some stimuli exhibit a single change, while others exhibit changes in different features at different times, leading to ambiguous segment boundaries and groupings.We first tested whether our subjects were able to correctly associate changes with musical features, to establish that their understanding of the stimuli was multidimensional and not purely holistic. Second, we tested whether an explicit instruction to focus on a feature increased the salience of boundaries marked by a change in that feature. Finally, we tested whether focusing on a feature would make groupings according to that feature preferable. To do so, we asked subjects to perform a distractor pattern-detection task that directed their attention to a particular feature. They then heard ambiguous stimuli, which had structure AAB and ABB with respect to two different features, and indicated their preferred grouping.The results showed that listeners were skilled at identifying changes, that correctly-directed attention boosted the salience of changes, and that focusing on a feature could indeed cause a listener to prefer one grouping over another. Whereas one’s level of musical training greatly impacted how one responded on the first two experiments, its impact was not significant in the third task, suggesting that attention is a general mechanism in guiding grouping preferences

Extraction of the πNN\pi NN coupling constant from NN scattering data

We reexamine Chew's method for extracting the $\pi NN$ coupling constant from np differential cross section measurements. Values for this coupling are extracted below 350 MeV, in the potential model region, and up to 1 GeV. The analyses to 1~GeV have utilized 55 data sets. We compare these results to those obtained via $\chi^2$ mapping techniques. We find that these two methods give consistent results which are in agreement with previous Nijmegen determinations.Comment: 12 pages of text plus 2 figures. Revtex file and postscript figures available via anonymous FTP at ftp://clsaid.phys.vt.edu/pub/n