3,484 research outputs found
A Continuation Method for Nash Equilibria in Structured Games
Structured game representations have recently attracted interest as models
for multi-agent artificial intelligence scenarios, with rational behavior most
commonly characterized by Nash equilibria. This paper presents efficient, exact
algorithms for computing Nash equilibria in structured game representations,
including both graphical games and multi-agent influence diagrams (MAIDs). The
algorithms are derived from a continuation method for normal-form and
extensive-form games due to Govindan and Wilson; they follow a trajectory
through a space of perturbed games and their equilibria, exploiting game
structure through fast computation of the Jacobian of the payoff function. They
are theoretically guaranteed to find at least one equilibrium of the game, and
may find more. Our approach provides the first efficient algorithm for
computing exact equilibria in graphical games with arbitrary topology, and the
first algorithm to exploit fine-grained structural properties of MAIDs.
Experimental results are presented demonstrating the effectiveness of the
algorithms and comparing them to predecessors. The running time of the
graphical game algorithm is similar to, and often better than, the running time
of previous approximate algorithms. The algorithm for MAIDs can effectively
solve games that are much larger than those solvable by previous methods
Hot Gas in the Galactic Thick Disk and Halo Near the Draco Cloud
This paper examines the ultraviolet and X-ray photons generated by hot gas in
the Galactic thick disk or halo in the Draco region of the northern hemisphere.
Our analysis uses the intensities from four ions, C IV, O VI, O VII, and O
VIII, sampling temperatures of ~100,000 to ~3,000,000 K. We measured the O VI,
O VII and O VIII intensities from FUSE and XMM-Newton data and subtracted off
the local contributions in order to deduce the thick disk/halo contributions.
These were supplemented with published C IV intensity and O VI column density
measurements. Our estimate of the thermal pressure in the O VI-rich thick
disk/halo gas, p_{th}/k = 6500^{+2500}_{-2600} K cm^{-3}, suggests that the
thick disk/halo is more highly pressurized than would be expected from
theoretical analyses. The ratios of C IV to O VI to O VII to O VIII,
intensities were compared with those predicted by theoretical models. Gas which
was heated to 3,000,000 K then allowed to cool radiatively cannot produce
enough C IV or O VI-generated photons per O VII or O VIII-generated photon.
Producing enough C IV and O VI emission requires heating additional gas to
100,000 < T < 1,000,000 K. However, shock heating, which provides heating
across this temperature range, overproduces O VI relative to the others.
Obtaining the observed mix may require a combination of several processes,
including some amount of shock heating, heat conduction, and mixing, as well as
radiative cooling of very hot gas.Comment: 10 pages, 2 figures. Accepted for publication in the Astrophysical
Journa
Lepidactylus triarticulatus N. Sp., A New Haustoriid Amphipod from the Northern Gulf of Mexico
A new haustoriid amphipod, Lepidactylus triarticulatus n. sp., from the northern Gulf of Mexico is described and illustrated. The known range is from northern Padre Island, Texas, to Grand Isle, Louisiana. The species is ecologically plastic. On surf-exposed sandy beaches it is most abundant at the highest intertidal levels, but in fine-grained sands of wave-sheltered localities it occurs throughout the intertidal region into shallow subtidal depths. In central Texas bays it has been collected subtidally at salinities as low as 10 ppt. There are differences in morphological details of peraeopod 7 between the intertidal and subtidal populations which we regard as an ecotypic variation. A provisional generic characterization is given for the genus Lepidactylus Say
System for the measurement of ultra-low stray light levels
An apparatus is described for measuring the effectiveness of stray light suppression light shields and baffle arrangements used in optical space experiments and large space telescopes. The light shield and baffle arrangement and a telescope model are contained in a vacuum chamber. A source of short, high-powered light energy illuminates portions of the light shield and baffle arrangement and reflects a portion of same to a photomultiplier tube by virtue of multipath scattering. The resulting signal is transferred to time-channel electronics timed by the firing of the high energy light source allowing time discrimination of the signal thereby enabling the light scattered and suppressed by the model to be distinguished from the walls and holders around the apparatus
The Origin of the Hot Gas in the Galactic Halo: Confronting Models with XMM-Newton Observations
We compare the predictions of three physical models for the origin of the hot
halo gas with the observed halo X-ray emission, derived from 26 high-latitude
XMM-Newton observations of the soft X-ray background between l=120\degr and
l=240\degr. These observations were chosen from a much larger set of
observations as they are expected to be the least contaminated by solar wind
charge exchange emission. We characterize the halo emission in the XMM-Newton
band with a single-temperature plasma model. We find that the observed halo
temperature is fairly constant across the sky (~1.8e6-2.3e6 K), whereas the
halo emission measure varies by an order of magnitude (~0.0005-0.006 cm^-6 pc).
When we compare our observations with the model predictions, we find that most
of the hot gas observed with XMM-Newton does not reside in isolated extraplanar
supernova remnants -- this model predicts emission an order of magnitude too
faint. A model of a supernova-driven interstellar medium, including the flow of
hot gas from the disk into the halo in a galactic fountain, gives good
agreement with the observed 0.4-2.0 keV surface brightness. This model
overpredicts the halo X-ray temperature by a factor of ~2, but there are a
several possible explanations for this discrepancy. We therefore conclude that
a major (possibly dominant) contributor to the halo X-ray emission observed
with XMM-Newton is a fountain of hot gas driven into the halo by disk
supernovae. However, we cannot rule out the possibility that the extended hot
halo of accreted material predicted by disk galaxy formation models also
contributes to the emission.Comment: 20 pages, 14 figures. New version accepted for publication in ApJ.
Changes include new section discussing systematic errors (Section 3.2),
improved method for characterizing our model spectra (4.2.2), changes to
discussion of other observations (5.1). Note that we can no longer rule out
possibility that extended hot halo of accreted material contributes to
observed halo emission (see 5.2.1
FUSE Detection of Galactic OVI Emission in the Halo above the Perseus Arm
Background observations obtained with the Far Ultraviolet Spectroscopic
Explorer (FUSE) toward l=95.4, b=36.1 show OVI 1032,1038 in emission. This
sight line probes a region of stronger-than-average soft X-ray emission in the
direction of high-velocity cloud Complex C above a part of the disk where
Halpha filaments rise into the halo. The OVI intensities, 1600+/-300
ph/s/cm^2/sr (1032A) and 800+/-300 ph/s/cm^2/sr (1038A), are the lowest
detected in emission in the Milky Way to date. A second sight line nearby
(l=99.3, b=43.3) also shows OVI 1032 emission, but with too low a
signal-to-noise ratio to obtain reliable measurements. The measured
intensities, velocities, and FWHMs of the OVI doublet and the CII* line at
1037A are consistent with a model in which the observed emission is produced in
the Galactic halo by hot gas ejected by supernovae in the Perseus arm. An
association of the observed gas with Complex C appears unlikely.Comment: accepted for publication in ApJL, 11 pages including 3 figure
An Effective Theory for Midgap States in Doped Spin Ladder and Spin-Peierls Systems: Liouville Quantum Mechanics
In gapped spin ladder and spin-Peierls systems the introduction of disorder,
for example by doping, leads to the appearance of low energy midgap states. The
fact that these strongly correlated systems can be mapped onto one dimensional
noninteracting fermions provides a rare opportunity to explore systems which
have both strong interactions and disorder. In this paper we show that the
statistics of the zero energy midgap wave functions in these models can be
effectively described by Liouville Quantum Mechanics. This enables us to
calculate the disorder averaged N-point correlation functions of these states
(the explicit calculation is performed for N=2,3). We find that whilst these
midgap states are typically weakly correlated, their disorder averaged
correlation are power law. This discrepancy arises because the correlations are
not self-averaging and averages of the wave functions are dominated by
anomalously strongly correlated configurations.Comment: 13 page latex fil
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