13,264 research outputs found
Convex Independence in Permutation Graphs
A set C of vertices of a graph is P_3-convex if every vertex outside C has at
most one neighbor in C. The convex hull \sigma(A) of a set A is the smallest
P_3-convex set that contains A. A set M is convexly independent if for every
vertex x \in M, x \notin \sigma(M-x). We show that the maximal number of
vertices that a convexly independent set in a permutation graph can have, can
be computed in polynomial time
Top Management Team Heterogeneity and SME Export Performance: Investigating the Role of Environmental Uncertainty
Research has generally found that firm competing in high uncertainty environments gain advantages from having diverse (heterogeneous) management teams (TMTs). Employing a national survey of 70 small- and medium-sized enterprises (SMEs), the present study examined whether TM T heterogeneity in functional background, international work experience, and foreign language proficiency had a stronger relationship with export performance for firms competing in high than those competing in low uncertainty environments. Results were generally weak when market and industry factors contributing to uncertainty were examined separately; however, additional analysis showed that the hypothesized relationship did exist when uncertainty was measured employing both factors simultaneously
Lightweight XML-based query, integration and visualization of distributed, multimodality brain imaging data
A need of many neuroimaging researchers is to integrate multimodality brain data that may be stored in separate databases. To address this need we have developed a framework that provides a uniform XML-based query interface across multiple online data sources. The development of this framework is driven by the need to integrate neurosurgical and neuroimaging data related to language. The data sources for the language studies are 1) a web-accessible relational database of neurosurgical cortical stimulation mapping data (CSM) that includes patient-specific 3-D coordinates of each stimulation site mapped to an MRI reconstruction of the patient brain surface; and 2) an XML database of fMRI and structural MRI data and analysis results, created automatically by a batch program we have embedded in SPM. To make these sources available for querying each is wrapped as an XML view embedded in a web service. A top level web application accepts distributed XQueries over the sources, which are dispatched to the underlying web services. Returned results can be displayed as XML, HTML, CSV (Excel format), a 2-D schematic of a parcellated brain, or a 3-D brain visualization. In the latter case the CSM patient-specific coordinates returned by the query are sent to a transformation web-service for conversion to normalized space, after which they are sent to our 3-D visualization program MindSeer, which is accessed via Java WebStart through a generated link. The anatomical distribution of pooled CSM sites can then be visualized using various surfaces derived from brain atlases. As this framework is further developed and generalized we believe it will have appeal for researchers who wish to query, integrate and visualize results across their own databases as well as those of collaborators
Applying Compactness Constraints to Seismic Traveltime Tomography
Tomographic imaging problems are typically ill-posed and often require the use of regularization techniques to guarantee a stable solution. Minimization of a weighted norm of model length is one commonly used secondary constraint. Tikhonov methods exploit low-order differential operators to select for solutions that are small, flat, or smooth in one or more dimensions. This class of regularizing functionals may not always be appropriate, particularly in cases where the anomaly being imaged is generated by a non-smooth spatial process. Timelapse imaging of flow-induced seismic velocity anomalies is one such case; flow features are often characterized by spatial compactness or connectivity. We develop a traveltime tomography algorithm which selects for compact solutions through application of model-space iteratively reweighted least squares. Our technique is an adaptation of minimum support regularization methods previously developed within the potential theory community. We emphasize the application of compactness constraints to timelapse datasets differenced in the data domain, a process which allows recovery of compact perturbations in model properties. We test our inversion algorithm on a simple synthetic dataset generated using a velocity model with several localized velocity anomalies. We then demonstrate the efficacy of the algorithm on a CO2 sequestration monitoring dataset acquired at the Frio pilot site. In both cases, the addition of compactness constraints improves image quality by reducing spatial smearing due to limited angular aperture in the acquisition geometry.Toksoz, M. NafiMassachusetts Institute of Technology. Earth Resources Laborator
Computation of 3D Frequency-Domain Waveform Kernals for c(x,y,z) Media
Seismic tomography, as typically practiced on both the exploration, crustal, and global scales, considers only the arrival times of selected sets of phases and relies primarily on WKBJ theory during inversion. Since the mid 1980’s, researchers have explored, largely on a theoretical level, the possibility of inverting the entire seismic record. Due to the ongoing advances in CPU performance, full waveform inversion is finally becoming feasible on select problems with promising results emerging from frequency-domain methods. However, frequency-domain techniques using sparse direct solvers are currently constrained by memory limitations in 3D where they exhibit a O(n4) worst-case bound on memory usage. We sidestep this limitation by using a hybrid approach, calculating frequency domain Green’s functions for the scalar wave equation by driving a high-order, time-domain, finite-difference (FDTD) code to steady state using a periodic source. The frequency-domain response is extracted using the phase sensitive detection (PSD) method recently developed by Nihei and Li (2006). The resulting algorithm has an O(n3) memory footprint and is amenable to parallelization in the space, shot, or frequency domains. We demonstrate this approach by generating waveform inversion kernels for fully c(x,y,z) models. Our test examples include a realistic VSP experiment using the geometry and velocity models obtained from a site in Western Wyoming, and a deep crustal reflection/refraction profile based on the LARSE II geometry and the SCEC community velocity model. We believe that our 3D solutions to the scalar Helmholtz equation, for models with upwards of 100 million degrees of freedom, are the largest examples documented in the open geophysical literature. Such results suggest that iterative 3D waveform inversion is an achievable goal in the near future.Shell GameChangerMassachusetts Institute of Technology. Earth Resources Laborator
Independent nurse medication provision: A mixed method study assessing impact on patients' experience, processes, and costs in sexual health clinics
BACKGROUND: Local services in the United Kingdom National Health Service enable autonomous provision of medication by nurses, supporting individual nurses to gain prescribing qualifications or by introducing local patient group directions. AIM: To compare nurse prescribing and patient group directions about clinic processes, patients' experiences, and costs from the perspectives of providers, nurses, and patients. DESIGN: Mixed methods, comparative case study in five urban sexual health services in the United Kingdom. METHODS: Data were collected from nurse prescribers, patient group direction users and their patients July 2015 to December 2016. Nurse questionnaires explored training (funding and methods). Nurses recorded consultation durations and support from other professionals in clinical diaries. Patient notes were reviewed to explore medication provision, appropriateness and safety; errors were judged by an expert panel. Patients completed satisfaction questionnaires about consultations and information about medications. RESULTS: Twenty-eight nurse prescribers and 67 patient group directions users took part; records of 1682 consultations were reviewed, with 1357 medications prescribed and 98.5% therapeutically appropriate. Most medication decisions were deemed safe (96.0% nurse prescribers, 98.7% patient group directions, Fisher's Exact Test p = .55). Errors were predominantly minor (55.6% nurse prescribers, 62.4% patient group directions) and related to documentation omissions (78.0%); no patients were harmed. Consultation durations and unplanned re-consultations were similar for both groups. Nurse prescribers sought assistance from colleagues less frequently (chi-squared = 46.748, df = 1, p 96%). CONCLUSIONS: Nurse medication provision by both nurse prescribers and patient group direction users is safe and associated with high patient satisfaction; effects on clinic processes and costs are similar. Undertaking the prescribing qualification involves independent study but may bring longer-term career progression to nurses
Octet Baryon Magnetic Moments in the Chiral Quark Model with Configuration Mixing
The Coleman-Glashow sum-rule for magnetic moments is always fulfilled in the
chiral quark model, independently of SU(3) symmetry breaking. This is due to
the structure of the wave functions, coming from the non-relativistic quark
model. Experimentally, the Coleman-Glashow sum-rule is violated by about ten
standard deviations. To overcome this problem, two models of wave functions
with configuration mixing are studied. One of these models violates the
Coleman-Glashow sum-rule to the right degree and also reproduces the octet
baryon magnetic moments rather accurately.Comment: 22 pages, RevTe
Radio-frequency discharges in Oxygen. Part 1: Modeling
In this series of three papers we present results from a combined
experimental and theoretical effort to quantitatively describe capacitively
coupled radio-frequency discharges in oxygen. The particle-in-cell Monte-Carlo
model on which the theoretical description is based will be described in the
present paper. It treats space charge fields and transport processes on an
equal footing with the most important plasma-chemical reactions. For given
external voltage and pressure, the model determines the electric potential
within the discharge and the distribution functions for electrons, negatively
charged atomic oxygen, and positively charged molecular oxygen. Previously used
scattering and reaction cross section data are critically assessed and in some
cases modified. To validate our model, we compare the densities in the bulk of
the discharge with experimental data and find good agreement, indicating that
essential aspects of an oxygen discharge are captured.Comment: 11 pages, 10 figure
The continuity of the inversion and the structure of maximal subgroups in countably compact topological semigroups
In this paper we search for conditions on a countably compact
(pseudo-compact) topological semigroup under which: (i) each maximal subgroup
in is a (closed) topological subgroup in ; (ii) the Clifford part
(i.e. the union of all maximal subgroups) of the semigroup is a
closed subset in ; (iii) the inversion is continuous; and (iv) the projection ,
, onto the subset of idempotents of ,
is continuous
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