1,220 research outputs found
On the random dynamics of Volterra quadratic operators
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugÀnglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.We consider random dynamical systems generated by a special class of Volterra quadratic stochastic operators on the simplex Sm-1. We prove that in contrast to the deterministic set-up the trajectories of the random dynamical system almost surely converge to one of the vertices of the simplex Sm-1, implying the survival of only one species. We also show that the minimal random point attractor of the system equals the set of all vertices. The convergence proof relies on a martingale-type limit theorem, which we prove in the appendix.DFG, GSC 14, Berlin Mathematical Schoo
The comet 17P/Holmes 2007 outburst: the early motion of the outburst material
Context. On October 24, 2007 the periodic comet 17P/Holmes underwent an
astonishing outburst that increased its apparent total brightness from
magnitude V\sim17 up to V\sim2.5 in roughly two days. We report on Wendelstein
0.8 m telescope (WST) photometric observations of the early evolution stages of
the outburst. Aims. We studied the evolution of the structure morphology, its
kinematic, and estimated the ejected dust mass. Methods. We analized 126 images
in the BVRI photometric bands spread between 26/10/2007 and 20/11/2007. The
bright comet core appeared well separated from that one of a quickly expanding
dust cloud in all the data, and the bulk of the latter was contained in the
field of view of our instrument. The ejected dust mass was derived on the base
of differential photometry on background stars occulted by the moving cloud.
Results. The two cores were moving apart from each other at a relative
projected constant velocity of (9.87 +/- 0.07) arcsec/day (0.135 +/-0.001
km/sec). In the inner regions of the dust cloud we observed a linear increase
in size at a mean constant velocity of (14.6+/-0.3) arcsec/day (0.200+/-0.004
km/sec). Evidence of a radial velocity gradient in the expanding cloud was also
found. Our estimate for the expanding coma's mass was of the order of 10^{-2}-1
comet's mass implying a significant disintegration event. Conclusions. We
interpreted our observations in the context of an explosive scenario which was
more probably originated by some internal instability processes, rather than an
impact with an asteroidal body. Due to the peculiar characteristics of this
event, further observations and investigations are necessary in order to
enlight the nature of the physical processes that determined it.Comment: 5 pages, 3 figures, A&A accepte
Introducing the brain erythropoietin circle to explain adaptive brain hardware upgrade and improved performance
Executive functions, learning, attention, and processing speed are imperative facets of cognitive performance, affected in neuropsychiatric disorders. In clinical studies on different patient groups, recombinant human (rh) erythropoietin (EPO) lastingly improved higher cognition and reduced brain matter loss. Correspondingly, rhEPO treatment of young rodents or EPO receptor (EPOR) overexpression in pyramidal neurons caused remarkable and enduring cognitive improvement, together with enhanced hippocampal long-term potentiation. The âbrain hardware upgradeâ, underlying these observations, includes an EPO induced ~20% increase in pyramidal neurons and oligodendrocytes in cornu ammonis hippocampi in the absence of elevated DNA synthesis. In parallel, EPO reduces microglia numbers and dampens their activity and metabolism as prerequisites for undisturbed EPO-driven differentiation of pre-existing local neuronal precursors. These processes depend on neuronal and microglial EPOR. This novel mechanism of powerful postnatal neurogenesis, outside the classical neurogenic niches, and on-demand delivery of new cells, paralleled by dendritic spine increase, let us hypothesize a physiological procognitive role of hypoxia-induced endogenous EPO in brain, which we imitate by rhEPO treatment. Here we delineate the brain EPO circle as working model explaining adaptive âbrain hardware upgradeâ and improved performance. In this fundamental regulatory circle, neuronal networks, challenged by motorcognitive tasks, drift into transient âfunctional hypoxiaâ, thereby triggering neuronal EPO/EPOR expression
Searching for O in the SMC:Constraints on Oxygen Chemistry at Low Metallicities
We present a 39 h integration with the Odin satellite on the ground-state
118.75 GHz line of O2 towards the region of strongest molecular emission in the
Small Magellanic Cloud. Our 3sigma upper limit to the O2 integrated intensity
of <0.049 K km/s in a 9'(160 pc) diameter beam corresponds to an upper limit on
the O2/H2 abundance ratio of <1.3E-6. Although a factor of 20 above the best
limit on the O2 abundance obtained for a Galactic source, our result has
interesting implications for understanding oxygen chemistry at sub-solar metal
abundances. We compare our abundance limit to a variety of astrochemical models
and find that, at low metallicities, the low O2 abundance is most likely
produced by the effects of photo-dissociation on molecular cloud structure.
Freeze-out of molecules onto dust grains may also be consistent with the
observed abundance limit, although such models have not yet been run at
sub-solar initial metallicities.Comment: 4 pages, accepted to A&A Letter
Differences in B cell regulation in DRB1 shared epitope positive and negative rheumatoid arthritis
Tracing the sites of obscured star formation in the Antennae galaxies with Herschel-PACS
FIR imaging of interacting galaxies allows locating even hidden sites of star
formation and measuring of the relative strength of nuclear and extra-nuclear
star formation. We want to resolve the star-forming sites in the nearby system
of the Antennae. Thanks to the unprecedented sharpness and depth of the PACS
camera onboard ESA's Herschel Space Observatory, it is possible for the first
time to achieve a complete assessment of individual star-forming knots in the
FIR with scan maps at 70, 100, and 160 um. We used clump extraction photometry
and SED diagnostics to derive the properties related to star-forming activity.
The PACS 70, 100, and 160 um maps trace the knotty structure of the most recent
star formation along an arc between the two nuclei in the overlap area. The
resolution of the starburst knots and additional multi-wavelength data allow
their individual star formation history and state to be analysed. In
particular, the brightest knot in the mid-infrared (K1), east of the southern
nucleus, exhibits the highest activity by far in terms of dust heating and star
formation rate, efficiency, and density. With only 2 kpc in diameter, this area
has a 10-1000 um luminosity, which is as high as that of our Milky Way. It
shows the highest deficiency in radio emission in the radio-to-FIR luminosity
ratio and a lack of X-ray emission, classifying it as a very young complex. The
brightest 100 and 160 um emission region (K2), which is close to the collision
front and consists of 3 knots, also shows a high star formation density and
efficiency and lack of X-ray emission in its most obscured part, but an excess
in the radio-to-FIR luminosity ratio. This suggests a young stage, too, but
different conditions in its interstellar medium. Our results provide important
checkpoints for numerical simulations of interacting galaxies when modelling
the star formation and stellar feedback.Comment: 4 pages, 4 figures, 2 tables (A&A Herschel special issue
On the Neutrality of Flowshop Scheduling Fitness Landscapes
Solving efficiently complex problems using metaheuristics, and in particular
local searches, requires incorporating knowledge about the problem to solve. In
this paper, the permutation flowshop problem is studied. It is well known that
in such problems, several solutions may have the same fitness value. As this
neutrality property is an important one, it should be taken into account during
the design of optimization methods. Then in the context of the permutation
flowshop, a deep landscape analysis focused on the neutrality property is
driven and propositions on the way to use this neutrality to guide efficiently
the search are given.Comment: Learning and Intelligent OptimizatioN Conference (LION 5), Rome :
Italy (2011
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