16,580 research outputs found
Recommended from our members
System approach to disparity estimation
A system approach to disparity estimation using dynamic programming is presented. The four step system can calculate a dense correspondence map between a stereo pair with parallel or
nonparallel camera geometry. Results are presented with CCIR 601 format stereo images
Rabi Oscillations in Landau-Quantized Graphene
We investigate the relation between the canonical model of quantum optics,
the Jaynes-Cummings Hamiltonian and Dirac fermions in quantizing magnetic
field. We demonstrate that Rabi oscillations are observable in the optical
response of graphene, providing us with a transparent picture about the
structure of optical transitions. While the longitudinal conductivity reveals
chaotic Rabi oscillations, the Hall component measures coherent ones. This
opens up the possibility of investigating a microscopic model of a few quantum
objects in a macroscopic experiment with tunable parameters.Comment: 5 pages, 4 figure
Lower Bound for the Fermi Level Density of States of a Disordered D-Wave Superconductor in Two Dimensions
We consider a disordered d--wave superconductor in two dimensions. Recently,
we have shown in an exact calculation that for a lattice model with a
Lorentzian distributed random chemical potential the quasiparticle density of
states at the Fermi level is nonzero. As the exact result holds only for the
special choice of the Lorentzian, we employ different methods to show that for
a large class of distributions, including the Gaussian distribution, one can
establish a nonzero lower bound for the Fermi level density of states. The fact
that the tails of the distributions are unimportant in deriving the lower bound
shows that the exact result obtained before is generic.Comment: 15 preprint pages, no figures, submitted to PR
Information filtering via preferential diffusion
Recommender systems have shown great potential to address information
overload problem, namely to help users in finding interesting and relevant
objects within a huge information space. Some physical dynamics, including heat
conduction process and mass or energy diffusion on networks, have recently
found applications in personalized recommendation. Most of the previous studies
focus overwhelmingly on recommendation accuracy as the only important factor,
while overlook the significance of diversity and novelty which indeed provide
the vitality of the system. In this paper, we propose a recommendation
algorithm based on the preferential diffusion process on user-object bipartite
network. Numerical analyses on two benchmark datasets, MovieLens and Netflix,
indicate that our method outperforms the state-of-the-art methods.
Specifically, it can not only provide more accurate recommendations, but also
generate more diverse and novel recommendations by accurately recommending
unpopular objects.Comment: 12 pages, 10 figures, 2 table
Laser-only adaptive optics achieves significant image quality gains compared to seeing-limited observations over the entire sky
Adaptive optics laser guide star systems perform atmospheric correction of
stellar wavefronts in two parts: stellar tip-tilt and high-spatial-order
laser-correction. The requirement of a sufficiently bright guide star in the
field-of-view to correct tip-tilt limits sky coverage. Here we show an
improvement to effective seeing without the need for nearby bright stars,
enabling full sky coverage by performing only laser-assisted wavefront
correction. We used Robo-AO, the first robotic AO system, to comprehensively
demonstrate this laser-only correction. We analyze observations from four years
of efficient robotic operation covering 15,000 targets and 42,000 observations,
each realizing different seeing conditions. Using an autoguider (or a
post-processing software equivalent) and the laser to improve effective seeing
independent of the brightness of a target, Robo-AO observations show a 39+/-19%
improvement to effective FWHM, without any tip-tilt correction. We also
demonstrate that 50% encircled-energy performance without tip-tilt correction
remains comparable to diffraction-limited, standard Robo-AO performance.
Faint-target science programs primarily limited by 50% encircled-energy (e.g.
those employing integral field spectrographs placed behind the AO system) may
see significant benefits to sky coverage from employing laser-only AO.Comment: Accepted for publication in The Astronomical Journal. 7 pages, 6
figure
Polytopality and Cartesian products of graphs
We study the question of polytopality of graphs: when is a given graph the
graph of a polytope? We first review the known necessary conditions for a graph
to be polytopal, and we provide several families of graphs which satisfy all
these conditions, but which nonetheless are not graphs of polytopes. Our main
contribution concerns the polytopality of Cartesian products of non-polytopal
graphs. On the one hand, we show that products of simple polytopes are the only
simple polytopes whose graph is a product. On the other hand, we provide a
general method to construct (non-simple) polytopal products whose factors are
not polytopal.Comment: 21 pages, 10 figure
The cross helicity at the solar surface by simulations and observations
The quasilinear mean-field theory for driven MHD turbulence leads to the
result that the observed cross helicity may directly yield the
magnetic eddy diffusivity \eta_{T} of the quiet Sun. In order to model the
cross helicity at the solar surface, magnetoconvection under the presence of a
vertical large-scale magnetic field is simulated with the nonlinear MHD code
NIRVANA. The very robust result of the calculations is that \simeq 2
independent of the applied magnetic field amplitude. The
correlation coefficient for the cross helicity is about 10%. Of similar
robustness is the finding that the rms value of the magnetic perturbations
exceeds the mean-field amplitude (only) by a factor of five. The characteristic
helicity speed u_{\eta} as the ratio of the eddy diffusivity and the density
scale height for an isothermal sound velocity of 6.6 km/s proves to be 1 km/s
for weak fields. This value well coincides with empirical results obtained from
the data of the HINODE satellite and the Swedish 1-m Solar Telescope (SST)
providing the cross helicity component . Both simulations and
observations thus lead to a numerical value of \eta_{T} \simeq 10^12 cm^2 /s as
characteristic for the surface of the quiet Sun.Comment: 6 pages, 6 figure
Epic Human Failure on June 30, 2013
Nineteen Prescott Fire Department, Granite Mountain Hot Shot (GMHS) wildland firefighters and supervisors (WFF), perished on the June 2013 Yarnell Hill Fire (YHF) in Arizona. The firefighters left their Safety Zone during forecast, outflow winds, triggering explosive fire behavior in drought-stressed
chaparral. Why would an experienced WFF Crew, leave ‘good black’ and travel downslope through a brush-filled chimney, contrary to their training and experience? An organized Serious Accident Investigation Team (SAIT) found, “… no indication of negligence, reckless actions, or violations of policy or protocol.” Despite this, many WFF professionals deemed the catastrophe, “… the final, fatal link, in a long chain of bad decisions with good outcomes.” This paper is a theoretical and realistic examination of plausible, faulty, human decisions with prior good outcomes; internal and external impacts, influencing the GMHS; and two explanations for this catastrophe: Individual Blame Logic and Organizational Function Logic, and proposed preventive mitigations
- …