1,846 research outputs found
Selection mechanisms affect volatility in evolving markets
Financial asset markets are sociotechnical systems whose constituent agents
are subject to evolutionary pressure as unprofitable agents exit the
marketplace and more profitable agents continue to trade assets. Using a
population of evolving zero-intelligence agents and a frequent batch auction
price-discovery mechanism as substrate, we analyze the role played by
evolutionary selection mechanisms in determining macro-observable market
statistics. In particular, we show that selection mechanisms incorporating a
local fitness-proportionate component are associated with high correlation
between a micro, risk-aversion parameter and a commonly-used macro-volatility
statistic, while a purely quantile-based selection mechanism shows
significantly less correlation.Comment: 9 pages, 7 figures, to appear in proceedings of GECCO 2019 as a full
pape
59Co-NMR Knight Shift of Superconducting Three-Layer NaxCoO2.yH2O
The superconducting state of NaxCoO2.yH2O with three CoO2 layers in a unit
cell has been studied by 59Co-NMR. The Knight shift measured for a peak of the
NMR spectra corresponding to the external magnetic field H along one of the
principal directions within the CoO2 plane, exhibits a rapid decrease with
decreasing temperature T below the superconducting transition temperature Tc,
indicating that the spin susceptibility is suppressed in the superconducting
phase, at least, for this field direction. Because differences of the
superconducting properties are rather small between this three-layer
NaxCoO2.yH2O and previously reported NaxCoO2.yH2O with two CoO2 layers within a
unit cell, the present result of the Knight shift studies indicates that the
Cooper pairs of the former system are in the singlet state as in the latter,
for which the spin susceptibility is suppressed for both directions of H
parallel and perpendicular to the CoO2 plane.Comment: 5 page
Magnetic and Metal-Insulator Transitions in beta-Na0.5CoO2 and gamma-K0.5CoO2 -NMR and Neutron Diffraction Studies-
Co-oxides beta-Na0.5CoO2 and gamma-K0.5CoO2 have been prepared by the Na
de-intercalation from alpha-NaCoO2 and by the floating-zone method,
respectively. It has been found that successive phase transitions take place at
temperatures Tc1 and Tc2 in both systems. The appearance of the internal
magnetic field at Tc1 with decreasing temperature T indicates that the
antiferromagnetic order exists at T < Tc1, as in gamma-Na0.5CoO2. For
beta-Na0.5CoO2, the transition temperatures and the NMR parameters determined
from the data taken for magnetically ordered state are similar to those of
gamma-Na0.5CoO2, indicating that the difference of the stacking ways of the
CoO2 layers between these systems do not significantly affect their physical
properties. For gamma-K0.5CoO2, the quantitative difference of the physical
quantities are found from those of beta- and gamma-Na0.5CoO2. The difference
between the values of Tci (i = 1 and 2) of these systems might be explained by
considering the distance between CoO2 layers.Comment: 8 pages, 14 figures, 1 Tabl
Spider fauna of semiarid eastern colorado agroecosystems: Diversity, abundance, and effects of crop intensification
Spiders are critical predators in agroecosystems. Crop management practices can influence predator density and diversity, which, in turn, can influence pest management strategies. Crop intensification is a sustainable agricultural technique that can enhance crop production although optimizing soil moisture. To date, there is no information on how crop intensification affects natural enemy populations, particularly spiders. This study had two objectives: to characterize the abundance and diversity of spiders in eastern Colorado agroecosystems, and to test the hypothesis that spider diversity and density would be higher in wheat (Triticum aestivum L.) in crop-intensified rotations compared with wheat in conventional rotations. We collected spiders through pitfall, vacuum, and lookdown sampling from 2002 to 2007 to test these objectives. Over 11,000 spiders in 19 families from 119 species were captured from all sampling techniques. Interestingly, the hunting spider guild represented 89% of the spider fauna captured from all sites with the families Gnaphosidae and Lycosidae representing 75% of these spiders. Compared with European agroecosystems, these agroecosystems had greater diversity, which can be beneficial for the biological control of pests. Overall, spider densities were low in these semiarid cropping systems, and crop intensification effects on spider densities were not evident at this scale. © 2013 Entomological Society of America
Grounding Bohmian Mechanics in Weak Values and Bayesianism
Bohmian mechanics (BM) is a popular interpretation of quantum mechanics in
which particles have real positions. The velocity of a point x in configuration
space is defined as the standard probability current j(x) divided by the
probability density P(x). However, this ``standard'' j is in fact only one of
infinitely many that transform correctly and satisfy \dot P + \del . j=0. In
this article I show that there is a unique j that can be determined
experimentally as a weak value using techniques that would make sense to a
classical physicist. Moreover, this operationally defined j equals the standard
j, so, assuming \dot x = j/P, the possible Bohmian paths can also be determined
experimentally from a large enough ensemble. Furthermore, this approach to
deriving BM singles out x as the hidden variable, because (for example) the
operationally defined momentum current is in general incompatible with the
evolution of the momentum distribution. Finally I discuss how, in this setting,
the usual quantum probabilities can be derived from a Bayesian standpoint, via
the principle of indifference.Comment: 11 page
Characterizing Young Brown Dwarfs using Low Resolution Near-IR Spectra
We present near-infrared (1.0-2.4 micron) spectra confirming the youth and
cool effective temperatures of 6 brown dwarfs and low mass stars with
circumstellar disks toward the Chamaeleon II and Ophiuchus star forming
regions. The spectrum of one of our objects indicates that it has a spectral
type of ~L1, making it one of the latest spectral type young brown dwarfs
identified to date. Comparing spectra of young brown dwarfs, field dwarfs, and
giant stars, we define a 1.49-1.56 micron H2O index capable of determining
spectral type to within 1 sub-type, independent of gravity. We have also
defined an index based on the 1.14 micron sodium feature that is sensitive to
gravity, but only weakly dependent on spectral type for field dwarfs. Our 1.14
micron Na index can be used to distinguish young cluster members (t <~ 5 Myr)
from young field dwarfs, both of which may have the triangular H-band continuum
shape which persists for at least tens of Myr. Using effective temperatures
determined from the spectral types of our objects along with luminosities
derived from near and mid-infrared photometry, we place our objects on the H-R
diagram and overlay evolutionary models to estimate the masses and ages of our
young sources. Three of our sources have inferred ages (t ~= 10-30 Myr)
significantly older than the median stellar age of their parent clouds (1-3
Myr). For these three objects, we derive masses ~3 times greater than expected
for 1-3 Myr old brown dwarfs with the bolometric luminosities of our sources.
The large discrepancies in the inferred masses and ages determined using two
separate, yet reasonable methods, emphasize the need for caution when deriving
or exploiting brown dwarf mass and age estimates.Comment: 11 pages, Accepted to Ap
First Fruits of the Spitzer Space Telescope: Galactic and Solar System Studies
This article provides a brief overview of the Spitzer Space Telescope and
discusses its initial scientific results on galactic and solar system science.Comment: Review article to appear in slightly different format in Vol.44 of
Annual Reviews of Astronomy and Astrophysics, 200
The very nearby M/T dwarf binary SCR 1845-6357
The recently discovered star SCR 1845-6357 is the first late M/T dwarf binary
discovered. SCR 1845 is a particular object due to its tight orbit (currently
around 4 AU) and its proximity to the Sun (3.85 pc). We present spatially
resolved VLT/NACO images and low resolution spectra of SCR 1845 in the J, H and
K near-infrared bands. Since the T dwarf companion, SCR 1845B, is so close to
the primary SCR 1845A, orbital motion is evident even within a year. Following
the orbital motion, the binary's mass can be measured accurately within a
decade, making SCR 1845B a key T-dwarf mass-luminosity calibrator. The NIR
spectra allow for accurate determination of spectral type and also for rough
estimates of the object's physical parameters. The spectral type of SCR 1845B
is determined by direct comparison of the flux calibrated JHK spectra with T
dwarf standard template spectra and also by NIR spectral indices obtained from
synthetic photometry. Constrained values for surface gravity, effective
temperature and metallicity are derived by comparison with model spectra. Our
data prove that SCR 1845B is a brown dwarf of spectral type T6 that is
co-moving with and therefore gravitationally bound to the M8.5 primary. Fitting
the NIR spectrum of SCR 1845B to model spectra yields an effective temperature
of about 950K and a surface gravity log(g)=5.1 (cgs) assuming solar
metallicity. Mass and age of SCR 1845B are in the range 40 to 50 Jupiter masses
and 1.8 to 3.1 Gyr.Comment: 5 pages, 4 figures, accepted for publication in Astronomy &
Astrophysic
CLOUDS search for variability in brown dwarf atmospheres
Context: L-type ultra-cool dwarfs and brown dwarfs have cloudy atmospheres
that could host weather-like phenomena. The detection of photometric or
spectral variability would provide insight into unresolved atmospheric
heterogeneities, such as holes in a global cloud deck.
Aims: It has been proposed that growth of heterogeneities in the global cloud
deck may account for the L- to T-type transition as brown dwarf photospheres
evolve from cloudy to clear conditions. Such a mechanism is compatible with
variability. We searched for variability in the spectra of five L6 to T6 brown
dwarfs in order to test this hypothesis.
Methods: We obtained spectroscopic time series using VLT/ISAAC, over
0.99-1.13um, and IRTF/SpeX for two of our targets, in J, H and K bands. We
search for statistically variable lines and correlation between those.
Results: High spectral-frequency variations are seen in some objects, but
these detections are marginal and need to be confirmed. We find no evidence for
large amplitude variations in spectral morphology and we place firm upper
limits of 2 to 3% on broad-band variability, on the time scale of a few hours.
The T2 transition brown dwarf SDSS J1254-0122 shows numerous variable features,
but a secure variability diagnosis would require further observations.
Conclusions: Assuming that any variability arises from the rotation of
patterns of large-scale clear and cloudy regions across the surface, we find
that the typical physical scale of cloud cover disruption should be smaller
than 5-8% of the disk area for four of our targets. The possible variations
seen in SDSS J1254-0122 are not strong enough to allow us to confirm the cloud
breaking hypothesis.Comment: 17 pages, 14 figures, accepted by A&
Distribution and abundance of fish and crayfish in a Waikato stream in relation to basin area
The aim of this study was to relate the longitudinal distribution of fish and crayfish to increasing basin area and physical site characteristics in the Mangaotama Stream, Waikato region, North Island, New Zealand. Fish and crayfish were captured with two-pass removal electroshocking at 11 sites located in hill-country with pasture, native forest, and mixed land uses within the 21.6 km2 basin. Number of fish species and lineal biomass of fish increased with increasing basin area, but barriers to upstream fish migration also influenced fish distribution; only climbing and non-migratory species were present above a series of small waterfalls. Fish biomass increased in direct proportion to stream width, suggesting that fish used much of the available channel, and stream width was closely related to basin area. Conversely, the abundance of crayfish was related to the amount of edge habitat, and therefore crayfish did not increase in abundance as basin area increased. Densities of all fish species combined ranged from 17 to 459 fish 100 m-2, and biomass ranged from 14 to 206 g m-2. Eels dominated the fish assemblages, comprising 85-100% of the total biomass; longfinned eels the majority of the biomass at most sites. Despite the open access of the lower sites to introduced brown trout, native species dominated all the fish communities sampled
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