333 research outputs found
Reduction and reconstruction of stochastic differential equations via symmetries
An algorithmic method to exploit a general class of infinitesimal symmetries
for reducing stochastic differential equations is presented and a natural
definition of reconstruction, inspired by the classical reconstruction by
quadratures, is proposed. As a side result the well-known solution formula for
linear one-dimensional stochastic differential equations is obtained within
this symmetry approach. The complete procedure is applied to several examples
with both theoretical and applied relevance
A compact ultrahigh-vacuum system for the in situ investigation of III/V semiconductor surfaces
A compact ultrahigh vacuum (UHV) system has been built to study growth and properties of III/V semiconductor surfaces and nanostructures. The system allows one to grow III/V semiconductor surfaces by molecular beam epitaxy (MBE) and analyze their surface by a variety of surface analysis techniques. The geometric structure is examined by scanning tunneling microscopy (STM), low-energy electron diffraction and reflection high-energy electron diffraction. The electronic properties of the surfaces are studied by angular resolved photoemission either in the laboratory using a helium discharge lamp or at the Berlin Synchrotron Radiation Facility BESSY. In order to meet the space restriction at BESSY the system dimensions are kept very small. A detailed description of the apparatus and the sample handling system is given. For the UHV-STM (Park Scientific Instruments, VP2) a new, versatile tip handling mechanism has been developed. It allows the transfer of tips out of the chamber and furthermore, the in situ tip cleaning by electron annealing. In addition, another more reliable in situ tip-preparation technique operating the STM in the field emission regime is described. The ability of the system is shown by an atomically resolved STM image of the c(4Ă4) reconstructed GaAs(001) surface
The VOISE Algorithm: a Versatile Tool for Automatic Segmentation of Astronomical Images
The auroras on Jupiter and Saturn can be studied with a high sensitivity and
resolution by the Hubble Space Telescope (HST) ultraviolet (UV) and
far-ultraviolet (FUV) Space Telescope spectrograph (STIS) and Advanced Camera
for Surveys (ACS) instruments. We present results of automatic detection and
segmentation of Jupiter's auroral emissions as observed by HST ACS instrument
with VOronoi Image SEgmentation (VOISE). VOISE is a dynamic algorithm for
partitioning the underlying pixel grid of an image into regions according to a
prescribed homogeneity criterion. The algorithm consists of an iterative
procedure that dynamically constructs a tessellation of the image plane based
on a Voronoi Diagram, until the intensity of the underlying image within each
region is classified as homogeneous. The computed tessellations allow the
extraction of quantitative information about the auroral features such as mean
intensity, latitudinal and longitudinal extents and length scales. These
outputs thus represent a more automated and objective method of characterising
auroral emissions than manual inspection.Comment: 9 pages, 7 figures; accepted for publication in MNRA
Unfolding the Hierarchy of Voids
We present a framework for the hierarchical identification and
characterization of voids based on the Watershed Void Finder. The Hierarchical
Void Finder is based on a generalization of the scale space of a density field
invoked in order to trace the hierarchical nature and structure of cosmological
voids. At each level of the hierarchy, the watershed transform is used to
identify the voids at that particular scale. By identifying the overlapping
regions between watershed basins in adjacent levels, the hierarchical void tree
is constructed. Applications on a hierarchical Voronoi model and on a set of
cosmological simulations illustrate its potential.Comment: 5 pages, 2 figure
Non-intersecting squared Bessel paths and multiple orthogonal polynomials for modified Bessel weights
We study a model of non-intersecting squared Bessel processes in the
confluent case: all paths start at time at the same positive value , remain positive, and are conditioned to end at time at . In
the limit , after appropriate rescaling, the paths fill out a
region in the -plane that we describe explicitly. In particular, the paths
initially stay away from the hard edge at , but at a certain critical
time the smallest paths hit the hard edge and from then on are stuck to
it. For we obtain the usual scaling limits from random matrix
theory, namely the sine, Airy, and Bessel kernels. A key fact is that the
positions of the paths at any time constitute a multiple orthogonal
polynomial ensemble, corresponding to a system of two modified Bessel-type
weights. As a consequence, there is a matrix valued
Riemann-Hilbert problem characterizing this model, that we analyze in the large
limit using the Deift-Zhou steepest descent method. There are some novel
ingredients in the Riemann-Hilbert analysis that are of independent interest.Comment: 59 pages, 11 figure
Only the Lonely: H I Imaging of Void Galaxies
Void galaxies, residing within the deepest underdensities of the Cosmic Web,
present an ideal population for the study of galaxy formation and evolution in
an environment undisturbed by the complex processes modifying galaxies in
clusters and groups, as well as provide an observational test for theories of
cosmological structure formation. We have completed a pilot survey for the HI
imaging aspects of a new Void Galaxy Survey (VGS), imaging 15 void galaxies in
HI in local (d < 100 Mpc) voids. HI masses range from 3.5 x 10^8 to 3.8 x 10^9
M_sun, with one nondetection with an upper limit of 2.1 x 10^8 M_sun. Our
galaxies were selected using a structural and geometric technique to produce a
sample that is purely environmentally selected and uniformly represents the
void galaxy population. In addition, we use a powerful new backend of the
Westerbork Synthesis Radio Telescope that allows us to probe a large volume
around each targeted galaxy, simultaneously providing an environmentally
constrained sample of fore- and background control sample of galaxies while
still resolving individual galaxy kinematics and detecting faint companions in
HI. This small sample makes up a surprisingly interesting collection of
perturbed and interacting galaxies, all with small stellar disks. Four galaxies
have significantly perturbed HI disks, five have previously unidentified
companions at distances ranging from 50 to 200 kpc, two are in interacting
systems, and one was found to have a polar HI disk. Our initial findings
suggest void galaxies are a gas-rich, dynamic population which present evidence
of ongoing gas accretion, major and minor interactions, and filamentary
alignment despite the surrounding underdense environment.Comment: 53 pages, 18 figures, accepted for publication in AJ. High resolution
available at http://www.astro.columbia.edu/~keejo/kreckel2010.pd
Recruiting Hard-to-Reach Subjects for Exercise Interventions: A Multi-Centre and Multi-Stage Approach Targeting General Practitioners and Their Community-Dwelling and Mobility-Limited Patients
The general practitioner (GP)âs practice appears to be an ideal venue for recruiting community-dwelling older adults with limited mobility. This study (Current Controlled Trials ISRCTN17727272) aimed at evaluating the recruiting process used for a multi-centre exercise intervention (HOMEfit). Each of six steps resulted in an absolute number of patients (N1âN6). Sex and age (for N4âN6) and reasons for dropping out were assessed. Patient database screening (N1âN3) at 15 GP practices yielded N1 = 5,990 patients aged 70 and above who had visited their GP within the past 6 months, N2 = 5,467 after exclusion of institutionalised patients, N3 = 1,545 patients eligible. Using a pre-defined limitation algorithm in order to conserve the practicesâ resources resulted in N4 = 1,214 patients (80.3 ± 5.6 years, 68% female), who were then officially invited to the final assessment of eligibility at the GPâs practice. N5 = 434 patients (79.5 ± 5.4 years, 69% female) attended the practice screening (n = 13 of whom had not received an official invitation). Finally, N6 = 209 (79.8 ± 5.2 years, 74% female) were randomised after they were judged eligible and had given their written informed consent to participate in the randomised controlled trial (overall recruitment rate: 4.4%). The general strategy of utilising a GPâs practice to recruit the target group proved beneficial. The data and experiences presented here can help planners of future exercise-intervention studies
Cavity evolution in relativistic self-gravitating fluids
We consider the evolution of cavities within spherically symmetric
relativistic fluids, under the assumption that proper radial distance between
neighboring fluid elements remains constant during their evolution (purely
areal evolution condition). The general formalism is deployed and solutions are
presented. Some of them satisfy Darmois conditions whereas others present
shells and must satisfy Israel conditions, on either one or both boundary
surfaces. Prospective applications of these results to some astrophysical
scenarios is suggested.Comment: 10 pages Revtex. To appear in Class. Quantum Grav
Arbitrage and deflators in illiquid markets
This paper presents a stochastic model for discrete-time trading in financial
markets where trading costs are given by convex cost functions and portfolios
are constrained by convex sets. The model does not assume the existence of a
cash account/numeraire. In addition to classical frictionless markets and
markets with transaction costs or bid-ask spreads, our framework covers markets
with nonlinear illiquidity effects for large instantaneous trades. In the
presence of nonlinearities, the classical notion of arbitrage turns out to have
two equally meaningful generalizations, a marginal and a scalable one. We study
their relations to state price deflators by analyzing two auxiliary market
models describing the local and global behavior of the cost functions and
constraints
Anaerobic degradation of 2-propanol: Laboratory and pilot-scale studies
The anaerobic degradation of 2-propanol, an important industrial solvent, was scaled-up from batch assays to a pilot expanded granular sludge bed (EGSB) reactor at 25 °C. Batch studies indicated that 2-propanol followed Haldane kinetics, with a maximum rate at 10 g COD Lâ1. Concentrations as high as 25 g COD Lâ1 did not inhibit the degradation of ethanol, a common co-solvent. Similar specific methanogenic activities (SMA) were obtained for water-solvent and water-brewery sludges (88 and 77 ml CH4 g-VSâ1 dâ1 at 5 g COD Lâ1). Continuous degradation showed a lag-phase of three weeks with water-brewery sludge. Increases in 2-propanol load from 0.05 to 0.18 kg COD kg-VSâ1 dâ1 caused a shift from the consumption of soluble matter to methane production, indicating polyhydroxybutyrates (PHB) accumulation. Conversely, smooth increases of up to 0.29 kg COD kg-VSâ1 dâ1 allowed 2-propanol degradation without PHB accumulation. The slowdown rate of 2-propanol-oxidizer and acetate-utilizing methanogen bacteria below 20 °C adversely impacted both removal and CH4 yield
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