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Search Results: 1 - 10 of 587391 matches for " A. P. Walker "
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A Fine Grain Configurable Logic Block for Self-checking FPGAs
P. K. Lala,A. Walker
VLSI Design , 2001, DOI: 10.1155/2001/83474
Abstract: This paper proposes a logic cell that can be used as a building block for Self-checking FPGAs. The proposed logic cell consists of two 2-to-1 multiplexers, three 4-to-1 multiplexers and a D flip-flop. The cell has been designed using Differential Cascode Voltage Switch Logic. It is self-checking for all single transistor stuck-on and stuck-off faults as well as stuck-at faults at the inputs of each multiplexers and the D flip-flop. The multiplexers and the D flip-flop provide either correct (complementary) output in the absence of above-mentioned faults; otherwise the outputs are identical.
Classical and quantum radiation reaction in conformally flat spacetime
A. Higuchi,P. J. Walker
Physics , 2009, DOI: 10.1103/PhysRevD.79.105023
Abstract: We investigate the physics of a charged scalar particle moving in conformally flat spacetime with the conformal factor depending only on time in the framework of quantum electrodynamics (QED). In particular, we show that the radiation-reaction force derived from QED agrees with the classical counterpart in the limit $\hbar \to 0$ using the fact that to lowest order in $\hbar$ the charged scalar field theory with mass $m$ in conformally flat spacetime with conformal factor $\Omega(t)$, which we call Model B, is equivalent to that in flat spacetime with a time-dependent mass $m\Omega(t)$, which we call Model A, at tree level in this limit. We also consider the one-loop QED corrections to these two models in the semi-classical approximation. We find nonzero one-loop corrections to the mass and Maxwell's equations in Model A at order $\hbar^{-1}$. This does not mean, however, that the corresponding one-loop corrections in Model B are nonzero because the equivalence of these models through a conformal transformation breaks down at one loop. We find that the one-loop corrections vanish in the limit $\hbar \to 0$ in Model B.
Preventing cardiovascular disease: A despondent view
Walker Alexander R. P.,Wadee Ahmed A.
Bulletin of the World Health Organization , 2002,
Global Constraints On Key Cosmological Parameters
G. Steigman,T. P. Walker,A. Zentner
Physics , 2000,
Abstract: Data from Type Ia supernovae, along with X-ray cluster estimates of the universal baryon fraction and Big Bang Nucleosynthesis (BBN) determinations of the baryon-to-photon ratio, are used to provide estimates of several global cosmological parameters at epochs near zero redshift. We show that our estimate of the present baryon density is in remarkably good agreement with that inferred from BBN at high redshift, provided the primordial abundance of deuterium is relatively low and the Universe is flat. We also compare these estimates to the baryon density at z = 1100 as inferred from the CMB angular power spectrum.
Learning Content Selection Rules for Generating Object Descriptions in Dialogue
P. W. Jordan,M. A. Walker
Computer Science , 2011, DOI: 10.1613/jair.1591
Abstract: A fundamental requirement of any task-oriented dialogue system is the ability to generate object descriptions that refer to objects in the task domain. The subproblem of content selection for object descriptions in task-oriented dialogue has been the focus of much previous work and a large number of models have been proposed. In this paper, we use the annotated COCONUT corpus of task-oriented design dialogues to develop feature sets based on Dale and Reiters (1995) incremental model, Brennan and Clarks (1996) conceptual pact model, and Jordans (2000b) intentional influences model, and use these feature sets in a machine learning experiment to automatically learn a model of content selection for object descriptions. Since Dale and Reiters model requires a representation of discourse structure, the corpus annotations are used to derive a representation based on Grosz and Sidners (1986) theory of the intentional structure of discourse, as well as two very simple representations of discourse structure based purely on recency. We then apply the rule-induction program RIPPER to train and test the content selection component of an object description generator on a set of 393 object descriptions from the corpus. To our knowledge, this is the first reported experiment of a trainable content selection component for object description generation in dialogue. Three separate content selection models that are based on the three theoretical models, all independently achieve accuracies significantly above the majority class baseline (17%) on unseen test data, with the intentional influences model (42.4%) performing significantly better than either the incremental model (30.4%) or the conceptual pact model (28.9%). But the best performing models combine all the feature sets, achieving accuracies near 60%. Surprisingly, a simple recency-based representation of discourse structure does as well as one based on intentional structure. To our knowledge, this is also the first empirical comparison of a representation of Grosz and Sidners model of discourse structure with a simpler model for any generation task.
River routing at the continental scale: use of globally-available data and an a priori method of parameter estimation
P. Naden,P. Broadhurst,N. Tauveron,A. Walker
Hydrology and Earth System Sciences (HESS) & Discussions (HESSD) , 1999,
Abstract: Two applications of a river routing model based on the observed river network and a linearised solution to the convective-diffusion equation are presented. One is an off-line application to part of the Amazon basin (catchment area 2.15 M km2) using river network data from the Digital Chart of the World and GCM-generated runoff at a grid resolution of 2.5 degrees latitude and 3.75 degrees longitude. The other application is to the Arkansas (409,000 km2) and Red River (125,500 km2) basins as an integrated component of a macro-scale hydrological model, driven by observed meteorology and operating on a 17 km grid. This second application makes use of the US EPA reach data to construct the river network. In both cases, a method of computing parameter values a priori has been applied and shows some success, although some interpretation is required to derive `correct' parameter values and further work is needed to develop guidelines for use of the method. The applications, however, do demonstrate the possibilities for applying the routing model at the continental scale, with globally-available data and a priori parameter estimation, and its value for validating GCM output against observed flows.
Validation of ACE-FTS v2.2 measurements of HCl, HF, CCl3F and CCl2F2 using space-, balloon- and ground-based instrument observations
E. Mahieu,P. Duchatelet,P. Demoulin,K. A. Walker
Atmospheric Chemistry and Physics Discussions , 2008,
Abstract: Hydrogen chloride (HCl) and hydrogen fluoride (HF) are respectively the main chlorine and fluorine reservoirs in the Earth's stratosphere. Their buildup resulted from the intensive use of man-made halogenated source gases, in particular CFC-11 (CCl3F) and CFC-12 (CCl2F2), during the second half of the 20th century. It is important to continue monitoring the evolution of these source gases and reservoirs, in support of the Montreal Protocol and also indirectly of the Kyoto Protocol. The Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) is a space-based instrument that has been performing regular solar occultation measurements of over 30 atmospheric gases since early 2004. In this validation paper, the HCl, HF, CFC-11 and CFC-12 version 2.2 profile data products retrieved from ACE-FTS measurements are evaluated. Volume mixing ratio profiles have been compared to observations made from space by MLS and HALOE, and from stratospheric balloons by SPIRALE, FIRS-2 and Mark-IV. Partial columns derived from the ACE-FTS data were also compared to column measurements from ground-based Fourier transform instruments operated at 12 sites. ACE-FTS data recorded from March 2004 to August 2007 have been used for the comparisons. These data are representative of a variety of atmospheric and chemical situations, with sounded air masses extending from the winter vortex to summer sub-tropical conditions. Typically, the ACE-FTS products are available in the 10–50 km altitude range for HCl and HF, and in the 7–20 and 7–25 km ranges for CFC-11 and CFC-12, respectively. For both reservoirs, comparison results indicate an agreement generally better than 5–10%, when accounting for the known offset affecting HALOE measurements of HCl and HF. Larger positive differences are however found for comparisons with single profiles from FIRS-2 and SPIRALE. For CFCs, the few coincident measurements available suggest that the differences probably remain within ±20%.
Parametric Study of Calibration Blackbody Uncertainty Using Design of Experiments  [PDF]
Nipa Phojanamongkolkij, Joe A. Walker, Richard P. Cageao, Martin G. Mlynczak, Joseph J. O’Connell, Rosemary R. Baize
Journal of Analytical Sciences, Methods and Instrumentation (JASMI) , 2012, DOI: 10.4236/jasmi.2012.23020
Abstract: NASA is developing the Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission to provide accurate measurements to substantially improve understanding of climate change. CLARREO will include a Reflected Solar (RS) Suite, an Infrared (IR) Suite, and a Global Navigation Satellite System-Radio Occultation (GNSS-RO). The IR Suite consists of a Fourier Transform Spectrometer (FTS) covering 5 to 50 micrometers (2000-200 cm-1 wavenumbers) and on-orbit calibration and verification systems. The IR instrument will use a cavity blackbody view and a deep space view for on-orbit calibration. The calibration blackbody and the verification system blackbody will both have Phase Change Cells (PCCs) to accurately provide a SI reference to absolute temperature. One of the most critical parts of obtaining accurate CLARREO IR scene measurements relies on knowing the spectral radiance output from the blackbody calibration source. The blackbody spectral radiance must be known with a low uncertainty, and the magnitude of the uncertainty itself must be reliably quantified. This study focuses on determining which parameters in the spectral radiance equation of the calibration blackbody are critical to the blackbody accuracy. Fourteen parameters are identified and explored. Design of Experiments (DOE) is applied to systematically set up an experiment (i.e., parameter settings and number of runs) to explore the effects of these 14 parameters. The experiment is done by computer simulation to estimate uncertainty of the calibration blackbody spectral radiance. Within the explored ranges, only 4 out of 14 parameters were discovered to be critical to the total uncertainty in blackbody radiance, and should be designed, manufactured, and/or controlled carefully. The uncertainties obtained by computer simulation are also compared to those obtained using the “Law of Propagation of Uncertainty”. The two methods produce statistically different uncertainties. Nevertheless, the differences are small and are not considered to be important. A follow-up study has been planned to examine the total combined uncertainty of the CLARREO IR Suite, with a total of 47 contributing parameters. The DOE method will help in identifying critical parameters that need to be effectively and efficiently designed to meet the stringent IR measurement accuracy requirements within the limited resources.
A Systematic Review of Proton Pump Inhibitors for the Prevention and Treatment of Preeclampsia and Gestational Hypertension  [PDF]
Manarangi De Silva, Fiona Brownfoot, Natalie J. Hannan, Susan P. Walker, Catherine A. Cluver, Anthea Lindquist, Stephen Tong, Roxanne Hastie
Open Journal of Obstetrics and Gynecology (OJOG) , 2019, DOI: 10.4236/ojog.2019.91003
Abstract: Background: Preeclampsia is a hypertensive disorder of pregnancy with a high rate of neonatal and maternal morbidity and mortality. The only definitive treatment is delivery. Through pre-clinical studies, proton pump inhibitors (PPIs), which are commonly and safely used in pregnancy, have been identified as potential therapeutic agents. Objective: To undertake a systematic review evaluating PPIs in the prevention and/or treatment of preeclampsia and gestational hypertension. Search strategy: Electronic databases were searched from inception to 2018. Search terms included preeclampsia, proton-pump inhibitors, pregnancy-induced hypertension, lansoprazole, rabeprazole, esomeprazole and omeprazole. Selection criteria: Studies were included if they were randomized control trials, case-control or cohort studies on human subjects. Case reports, review articles, opinion pieces and conference abstracts were excluded as well as studies with no or inappropriate control arms.
Limits to Interstellar C_4 and C_5 Towards zeta Ophiuchi
John P. Maier,Gordon A. H. Walker,David A. Bohlender
Physics , 2001, DOI: 10.1086/337965
Abstract: We have made a sensitive search for the origin bands in the known electronic transitions of the linear carbon chains C_4 and C_5 at 3789 and 5109 A towards zeta Oph (A_V <= 1). The incentive was a recent detection of C_3 in this interstellar cloud with a column density of 1.6 x 10^12 cm^-2 plus the availability of laboratory gas phase spectra of C_4 and C_5. Further, some models of diffuse interstellar clouds predict that the abundance of these latter species should be within an order of magnitude of C_3. Despite achieving S/N of 2300 to 2600 per pixel at a resolution of ~110,000, the searches were negative, leading to 3 sigma upper limits to the column density of N(C_5) = 2 x 10^11 cm^-2 and N(C_4) = 4 x 10^12-13 cm^-2 where these values rely on theoretically calculated oscillator strengths. The implication of these limits are discussed on the choice of molecules for study in future attempts to identify the carriers of the stronger diffuse interstellar bands.
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