9,015 research outputs found
Threshold effects for two pathogens spreading on a network
Diseases spread through host populations over the networks of contacts
between individuals, and a number of results about this process have been
derived in recent years by exploiting connections between epidemic processes
and bond percolation on networks. Here we investigate the case of two pathogens
in a single population, which has been the subject of recent interest among
epidemiologists. We demonstrate that two pathogens competing for the same hosts
can both spread through a population only for intermediate values of the bond
occupation probability that lie above the classic epidemic threshold and below
a second higher value, which we call the coexistence threshold, corresponding
to a distinct topological phase transition in networked systems.Comment: 5 pages, 2 figure
Satellite power system: Concept development and evaluation program. Volume 3: Power transmission and reception. Technical summary and assessment
Efforts in the DOE/NASA concept development and evaluation program are discussed for the solar power satellite power transmission and reception system. A technical summary is provided together with a summary of system assessment activities. System options and system definition drivers are described. Major system assessment activities were in support of the reference system definition, solid state system studies, critical technology supporting investigations, and various system and subsystem tradeoffs. These activities are described together with reference system updates and alternative concepts for each of the subsystem areas. Conclusions reached as a result of the numerous analytical and experimental evaluations are presented. Remaining issues for a possible follow-on program are identified
Physician’s Ability to Manually Detect Isolated Elevation in Leg Intracompartmental Pressure.
Background: Serial physical examination is recommended for patients for whom there is a high index of suspicion for compartment syndrome. This examination is more difficult when performed on an obtunded patient and relies on the sensitivity of manual palpation to detect compartment firmness—a direct manifestation of increased intracompartmental pressure. This study was performed to establish the sensitivity of manual palpation for detecting critical pressure elevations in the leg compartments most frequently involved in clinical compartment syndrome.
Methods: Reproducible, sustained elevation of intracompartmental pressure was established in fresh cadaver leg specimens. Pressures tested included 20 and 40 mm Hg (negative controls) and 60 and 80 mm Hg (considered to be consistent with a compartment syndrome). Each leg served as an internal control, with three compartments having a noncritical pressure elevation. Orthopaedic residents and faculty were individually invited to manually palpate the leg with a known compartment pressure and to answer the following questions: (1) Is there a compartment syndrome? (2) In which compartment or compartments do you believe the pressure is elevated, if at all? (3) Describe your examination findings as soft, compressible, or firm.
Results: When a true-positive result was considered to be the correct detection of an elevation of intracompartmental pressures and correct identification of the compartment with the elevated pressure, the sensitivity of manual palpation was 24%, the specificity was 55%, the positive predictive value was 19%, and the negative predictive value was 63%. With increasing intracompartmental pressure, fasciotomy was recommended with a higher frequency (19% when the pressure was 20 mm Hg, 28% when it was 40 mm Hg, 50% when it was 60 mm Hg, and 60% when it was 80 mm Hg). When a true-positive result of manual palpation was considered to be an appropriate recommendation of fasciotomy, regardless of the ability of the examiner to correctly localize the compartment with the critical pressure elevation, the sensitivity was 54%,the specificity was 76%, the positive predictive value was 70%, and the negative predictive value was 63%.
Conclusions: Manual detection of compartment firmness associated with critical elevations in intracompartmentalpressure is poor
Obstacle stepping in patients with Parkinson's disease: Complexity does influence performance
Patients with Parkinson's disease (PD) have difficulties in performing complex bimanual movements. Here we have examined acquisition and performance of a bilateral obstacle stepping task to see whether these difficulties are also present during bipedal movements. Subjects had to minimize foot clearance when repeatedly stepping on a treadmill over randomly approaching obstacles on either side. The subjects had full vision and received acoustic feedback information about task performance. Foot clearance improved in healthy and PD subjects during the acquisition of the task. However, PD subjects showed a slower improvement and achieved a poorer performance level. Thus, in contrast to unilateral obstacle stepping, where no deficits in performance after task repetition were found in PD subjects, bilateral obstacle stepping was poorer in these subjects compared to healthy subjects. The present results extend findings from upper to lower limb movements, namely that PD subjects have difficulties in the performance of bilateral motor task
At Risk Students at Urban and Rural Community Colleges
Research conducted at one urban and one rural community college illustrated factors that lead to persistence in completing educational goals, what specific factors put the students at risk, and how formal and informal actions taken at community colleges can improve successful completion for first generation college students living in poverty
CYK Tensors, Maxwell Field and Conserved Quantities for Spin-2 Field
Starting from an important application of Conformal Yano--Killing tensors for
the existence of global charges in gravity, some new observations at \scri^+
are given. They allow to define asymptotic charges (at future null infinity) in
terms of the Weyl tensor together with their fluxes through \scri^+. It
occurs that some of them play a role of obstructions for the existence of
angular momentum.
Moreover, new relations between solutions of the Maxwell equations and the
spin-2 field are given. They are used in the construction of new conserved
quantities which are quadratic in terms of the Weyl tensor. The obtained
formulae are similar to the functionals obtained from the
Bel--Robinson tensor.Comment: 20 pages, LaTe
Experimental Test of a Two-dimensional Approximation for Dielectric Microcavities
Open dielectric resonators of different shapes are widely used for the
manufacture of microlasers. A precise determination of their resonance
frequencies and widths is crucial for their design. Most microlasers have a
flat cylindrical geometry, and a two-dimensional approximation, the so-called
method of the effective index of refraction, is commonly employed for numerical
calculations. Our aim has been an experimental test of the precision and
applicability of a model based on this approximation. We performed very
thorough and accurate measurements of the resonance frequencies and widths of
two passive circular dielectric microwave resonators and found significant
deviations from the model predictions. From this we conclude that the model
generally fails in the quantitative description of three-dimensional dielectric
resonators.Comment: 10 pages, 13 figure
Quantum Chaotic Scattering in Microwave Resonators
In a frequency range where a microwave resonator simulates a chaotic quantum
billiard, we have measured moduli and phases of reflection and transmission
amplitudes in the regimes of both isolated and of weakly overlapping resonances
and for resonators with and without time-reversal invariance. Statistical
measures for S-matrix fluctuations were determined from the data and compared
with extant and/or newly derived theoretical results obtained from the
random-matrix approach to quantum chaotic scattering. The latter contained a
small number of fit parameters. The large data sets taken made it possible to
test the theoretical expressions with unprecedented accuracy. The theory is
confirmed by both, a goodness-of-fit-test and the agreement of predicted values
for those statistical measures that were not used for the fits, with the data
Encircling an Exceptional Point
We calculate analytically the geometric phases that the eigenvectors of a
parametric dissipative two-state system described by a complex symmetric
Hamiltonian pick up when an exceptional point (EP) is encircled. An EP is a
parameter setting where the two eigenvalues and the corresponding eigenvectors
of the Hamiltonian coalesce. We show that it can be encircled on a path along
which the eigenvectors remain approximately real and discuss a microwave cavity
experiment, where such an encircling of an EP was realized. Since the
wavefunctions remain approximately real, they could be reconstructed from the
nodal lines of the recorded spatial intensity distributions of the electric
fields inside the resonator. We measured the geometric phases that occur when
an EP is encircled four times and thus confirmed that for our system an EP is a
branch point of fourth order.Comment: RevTex 4.0, four eps-figures (low resolution
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