8,858 research outputs found
Generic Conditions for Forecast Dominance
Recent studies have analyzed whether one forecast method dominates another
under a class of consistent scoring functions. While the existing literature
focuses on empirical tests of forecast dominance, little is known about the
theoretical conditions under which one forecast dominates another. To address
this question, we derive a new characterization of dominance among forecasts of
the mean functional. We present various scenarios under which dominance occurs.
Unlike existing results, our results allow for the case that the forecasts'
underlying information sets are not nested, and allow for uncalibrated
forecasts that suffer, e.g., from model misspecification or parameter
estimation error. We illustrate the empirical relevance of our results via data
examples from finance and economics
Upper limit on the critical strength of central potentials in relativistic quantum mechanics
In the context of relativistic quantum mechanics, where the Schr\"odinger
equation is replaced by the spinless Salpeter equation, we show how to
construct a large class of upper limits on the critical value,
, of the coupling constant, , of the central potential,
. This critical value is the value of for which a first
-wave bound state appears.Comment: 8 page
Soft X-ray background fluctuations and large-scale structure in the Universe
We have studied the fluctuations of the soft (0.9-2 keV) X-ray background
intensity for ~10 arcmin and ~2 arcmin beam sizes, using 80 high galactic
latitude medium-deep images from the ROSAT position sensitive proportional
counter (PSPC). These fluctuations are dominated (and well reproduced) by
confusion noise produced by sources unresolved with the beam sizes we used. We
find no evidence for any excess fluctuations which could be attributed to
source clustering. The 95 per cent confidence upper limits on excess
fluctuations dIclus are: dIclus/Ixrb_10 arcmin<~ 0.12, dIclus/Ixrb_2 arcmin
<~0.07. We have checked the possibility that low surface brightness extended
objects (like groups or clusters of galaxies) may have a significant
contribution to excess fluctuations, finding that they are not necessary to fit
the distribution of fluctuations, and obtaining an upper limit on the surface
density for this type of source. Standard Cold Dark Matter models would produce
dIclus/Ixrb larger than the above limits for any value of the density of the
Universe Omega=0.1-1, unless the bias parameter of the X-ray emitting matter is
smaller than unity, or an important fraction of the sources of the soft X-ray
background (~30 per cent) is at redshifts z>1. Limits on the 2-10 keV excess
fluctuations are also considered, showing that X-ray sources in that band have
to be at redshifts z>1 unless Omega>0.4. Finally, if the spatial correlation
function of the sources that produce these excess fluctuations is instead a
power law, the density contrast drho/rho implied by the excess fluctuations
reveals that the Universe is smooth and linear on scales of tens of Mpc, while
it can be highly non-linear on scales ~1 Mpc.Comment: 10 pages, LaTeX file, epsf.sty and 7 postscript figures. To appear in
MNRAS. Fig. 7 replaced, some references improved, a few corrections to the
tex
Recommended from our members
Listeria monocytogenes cell-to-cell spread in epithelia is heterogeneous and dominated by rare pioneer bacteria.
Listeria monocytogenes hijacks host actin to promote its intracellular motility and intercellular spread. While L. monocytogenes virulence hinges on cell-to-cell spread, little is known about the dynamics of bacterial spread in epithelia at a population level. Here, we use live microscopy and statistical modeling to demonstrate that L. monocytogenes cell-to-cell spread proceeds anisotropically in an epithelial monolayer in culture. We show that boundaries of infection foci are irregular and dominated by rare pioneer bacteria that spread farther than the rest. We extend our quantitative model for bacterial spread to show that heterogeneous spreading behavior can improve the chances of creating a persistent L. monocytogenes infection in an actively extruding epithelium. Thus, our results indicate that L. monocytogenes cell-to-cell spread is heterogeneous, and that rare pioneer bacteria determine the frontier of infection foci and may promote bacterial infection persistence in dynamic epithelia. Editorial note:This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter)
Design of a middleware for QoS-aware distribution transparent content delivery
Developers of distributed multimedia applications face a diversity of multimedia formats, streaming platforms and streaming protocols. Furthermore, support for end-to-end quality-of-service (QoS) is a crucial factor for the development of future distributed multimedia systems. This paper discusses the architecture, design and implementation of a QoS-aware middleware platform for content delivery. The platform supports the development of distributed multimedia applications and can deliver content with QoS guarantees. QoS support is offered by means of an agent infrastructure for QoS negotiation and enforcement. Properties of content are represented using a generic content representation model described using the OMG Meta Object Facility (MOF) model. A content delivery framework manages stream paths for content delivery despite differences in streaming protocols and content encoding. The integration of the QoS support, content representation and content delivery framework results in a QoS-aware middleware that enables representation transparent and location transparent delivery of content
The effects of a comptonizing corona on the appearance of the reflection components in accreting black hole spectra
We discuss the effects of a comptonizing corona on the appearance of the
reflection components, and in particular of the reflection hump, in the X-rays
spectra of accreting black holes. Indeed, in the framework of a thermal corona
model, we expect that part (or even all, depending on the coronal covering
factor) of the reflection features should cross the hot plasma, and thus suffer
Compton scattering, before being observed. We have studied in detail the
dependence of these effects on the physical (i.e. temperature and optical
depth) and geometrical (i.e. inclination angle) parameters of the corona,
concentrating on the slab geometry . Due to the smoothing and shifting towards
high energies of the comptonized reflection hump, the main effects on the
emerging spectra appear above 100 keV. We have also investigated the importance
of such effects on the interpretation of the results obtained with the standard
fitting procedures. We found that fitting Comptonization models, taking into
account comptonized reflection, by the usual cut-off power law + uncomptonized
reflection model, may lead to an underestimation of the reflection
normalization and an overestimation of the high energy cut-off. We discuss and
illustrate the importance of these effects by analysing recent observational
results as those of the galaxy NGC 4258. We also find that the comptonizing
corona can produce and/or emphasize correlations between the reflection
features characteristics (like the iron line equivalent width or the covering
fraction) and the X-ray spectral index similar to those recently reported in
the literature. We also underline the importance of these effects when dealing
with accurate spectral fitting of the X-ray background.Comment: 11 pages, 14 figures accepted for publication in MNRAS. Version
printable on US 8.5x11 pape
Stellar streams as gravitational experiments I. The case of Sagittarius
Tidal streams of disrupting dwarf galaxies orbiting around their host galaxy
offer a unique way to constrain the shape of galactic gravitational potentials.
Such streams can be used as leaning tower gravitational experiments on galactic
scales. The most well motivated modification of gravity proposed as an
alternative to dark matter on galactic scales is Milgromian dynamics (MOND),
and we present here the first ever N-body simulations of the dynamical
evolution of the disrupting Sagittarius dwarf galaxy in this framework. Using a
realistic baryonic mass model for the Milky Way, we attempt to reproduce the
present-day spatial and kinematic structure of the Sagittarius dwarf and its
immense tidal stream that wraps around the Milky Way. With very little freedom
on the original structure of the progenitor, constrained by the total
luminosity of the Sagittarius structure and by the observed stellar mass-size
relation for isolated dwarf galaxies, we find reasonable agreement between our
simulations and observations of this system. The observed stellar velocities in
the leading arm can be reproduced if we include a massive hot gas corona around
the Milky Way that is flattened in the direction of the principal plane of its
satellites. This is the first time that tidal dissolution in MOND has been
tested rigorously at these mass and acceleration scales.Comment: 12 pages, 11 figures accepted for publication in A&A. The movie of
Fig 6 can be watched at
http://astro.unistra.fr/fileadmin/upload/DUN/observatoire/Images/GFThomas_MONDSgrstream_movie.mp
Continuation or Reorientation - What Future for European Integration?
On the 25th of March the European Union celebrates the 50th anniversary of the Treaty
of Rome by which the European Economic Community was established; the
Netherlands being one of the founding members. Until this very day, albeit in revised
form, this Treaty still constitutes the very centrepiece of the post-war European
supranational legal order.
Anniversaries are a time of celebration, a time of praise of previous accomplishments
and encouragements to keep up the good work. I am confident that the Informal
Meeting of the Heads of State or Government of the Member States planned for the
25th of March will do an excellent job in issuing a statement to that effect.
Yet, these celebrations come at an awkward time for Europe: there is talk of a
constitutional crisis and of a rift that goes through Europe caused by the Treaty
establishing a Constitution for Europe. With the negative referenda in the Netherlands
and France the destiny of this Treaty remains unclear. The initial disbelief triggered
by the rejection of this European prestige project by the citizens of two of the
founding members of the European Communities has given way for a certain
resignation regarding the current state and the future perspectives for Europe. Taken
aback were especially those who consider the Constitutional Treaty a substantial step
forward. For some the importance of this project has existential proportions, as its
failure is thought to jeopardize the future of European integration as a whole.Rede uitgesproken bij de aanvaarding van het ambt van Hoogleraar Recht van de Europese Unie aan de Faculteit der Rechtsgeleerdheid aan de Erasmus Universiteit Rotterdam.
In zijn oratie gaat Amtenbrink onder andere in op de organisatie van het Europees Parlement, waarin de burgers per land zijn vertegenwoordigd. Hij is van mening dat het huidige juridische kader niet voldoet aan de democratische grondslag, omdat het geen proportionele vertegenwoordiging van de burgers van de Europese Unie waarborgt. Bovendien is het Europees Parlement op belangrijke beleidsterreinen nog steeds nauwelijks betrokken bij de besluitvorming. Ook in de voorgestelde Europese Grondwet worden deze tekortkomingen niet opgelost.
Amtenbrink oordeelt dat voor de vooruitgang van de Europese Unie en vooral voor de beoogde verdere uitbreiding een grondige heroriëntatie noodzakelijk is. Anders dreigen burgers steeds meer in verzet te komen tegen Europa. Bij een reorganisatie van de huidige Europese instellingen dient ook buiten de gebaande paden gedacht te worden. Als mogelijke oplossingen stelt Amtenbrink voor een duidelijke scheiding tussen vertegenwoordiging van burgers en van lidstaten en de introductie van Europese verkiezingen gebaseerd op een Europees stembiljet
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