139,505 research outputs found
Learned versus Hand-Designed Feature Representations for 3d Agglomeration
For image recognition and labeling tasks, recent results suggest that machine
learning methods that rely on manually specified feature representations may be
outperformed by methods that automatically derive feature representations based
on the data. Yet for problems that involve analysis of 3d objects, such as mesh
segmentation, shape retrieval, or neuron fragment agglomeration, there remains
a strong reliance on hand-designed feature descriptors. In this paper, we
evaluate a large set of hand-designed 3d feature descriptors alongside features
learned from the raw data using both end-to-end and unsupervised learning
techniques, in the context of agglomeration of 3d neuron fragments. By
combining unsupervised learning techniques with a novel dynamic pooling scheme,
we show how pure learning-based methods are for the first time competitive with
hand-designed 3d shape descriptors. We investigate data augmentation strategies
for dramatically increasing the size of the training set, and show how
combining both learned and hand-designed features leads to the highest
accuracy
Reduced magnetohydrodynamic theory of oblique plasmoid instabilities
The three-dimensional nature of plasmoid instabilities is studied using the
reduced magnetohydrodynamic equations. For a Harris equilibrium with guide
field, represented by \vc{B}_o = B_{po} \tanh (x/\lambda) \hat{y} + B_{zo}
\hat{z}, a spectrum of modes are unstable at multiple resonant surfaces in the
current sheet, rather than just the null surface of the polodial field , which is the only resonant surface in 2D or in
the absence of a guide field. Here is the asymptotic value of the
equilibrium poloidal field, is the constant equilibrium guide field,
and is the current sheet width. Plasmoids on each resonant surface
have a unique angle of obliquity . The resonant
surface location for angle is x_s = - \lambda \arctanh (\tan \theta
B_{zo}/B_{po}), and the existence of a resonant surface requires . The most unstable angle is oblique, i.e. and , in the constant- regime, but parallel, i.e.
and , in the nonconstant- regime. For a fixed angle
of obliquity, the most unstable wavenumber lies at the intersection of the
constant- and nonconstant- regimes. The growth rate of this mode is
, in which
, is the Alfv\'{e}n speed, is the current sheet
length, and is the Lundquist number. The number of plasmoids scales as .Comment: 9 pages, 8 figures, to be published in Physics of Plasma
Duality and phase diagram of one dimensional transport
The observation of duality by Mukherji and Mishra in one dimensional
transport problems has been used to develop a general approach to classify and
characterize the steady state phase diagrams. The phase diagrams are determined
by the zeros of a set of coarse-grained functions without the need of detailed
knowledge of microscopic dynamics. In the process, a new class of
nonequilibrium multicritical points has been identified.Comment: 6 pages, 2 figures (4 eps files
Anisotropic Cosmological Models with Energy Density Dependent Bulk Viscosity
An analysis is presented of the Bianchi type I cosmological models with a
bulk viscosity when the universe is filled with the stiff fluid
while the viscosity is a power function of the energy density, such as . Although the exact solutions are obtainable only when the
is an integer, the characteristics of evolution can be clarified for the
models with arbitrary value of . It is shown that, except for the
model that has solutions with infinite energy density at initial state, the
anisotropic solutions that evolve to positive Hubble functions in the later
stage will begin with Kasner-type curvature singularity and zero energy density
at finite past for the models, and with finite Hubble functions and
finite negative energy density at infinite past for the models. In the
course of evolution, matters are created and the anisotropies of the universe
are smoothed out. At the final stage, cosmologies are driven to infinite
expansion state, de Sitter space-time, or Friedman universe asymptotically.
However, the de Sitter space-time is the only attractor state for the
models. The solutions that are free of cosmological singularity for any finite
proper time are singled out. The extension to the higher-dimensional models is
also discussed
Degenerate Fermi gas in a combined harmonic-lattice potential
In this paper we derive an analytic approximation to the density of states
for atoms in a combined optical lattice and harmonic trap potential as used in
current experiments with quantum degenerate gases. We compare this analytic
density of states to numerical solutions and demonstrate its validity regime.
Our work explicitly considers the role of higher bands and when they are
important in quantitative analysis of this system. Applying our density of
states to a degenerate Fermi gas we consider how adiabatic loading from a
harmonic trap into the combined harmonic-lattice potential affects the
degeneracy temperature. Our results suggest that occupation of excited bands
during loading should lead to more favourable conditions for realizing
degenerate Fermi gases in optical lattices.Comment: 11 pages, 9 figure
Josephson (001) tilt grain boundary junctions of high temperature superconductors
We calculate the critical current across in-plane (001) tilt grain
boundary junctions of high temperature superconductors. We solve for the
electronic states corresponding to the electron-doped cuprates, two slightly
different hole-doped cuprates, and an extremely underdoped hole-doped cuprate
in each half-space, and weakly connect the two half-spaces by either specular
or random quasiparticle tunneling. We treat symmetric, straight, and fully
asymmetric junctions with s-, extended-s-, or d-wave order
parameters. For symmetric junctions with random grain boundary tunneling, our
results are generally in agreement with the Sigrist-Rice form for ideal
junctions that has been used to interpret ``phase-sensitive'' experiments
consisting of such in-plane grain boundary junctions. For specular grain
boundary tunneling across symmetric juncitons, our results depend upon the
Fermi surface topology, but are usually rather consistent with the random facet
model of Tsuei {\it et al.} [Phys. Rev. Lett. {\bf 73}, 593 (1994)]. Our
results for asymmetric junctions of electron-doped cuparates are in agreement
with the Sigrist-Rice form. However, ou resutls for asymmetric junctions of
hole-doped cuprates show that the details of the Fermi surface topology and of
the tunneling processes are both very important, so that the
``phase-sensitive'' experiments based upon the in-plane Josephson junctions are
less definitive than has generally been thought.Comment: 13 pages, 10 figures, resubmitted to PR
Semileptonic Meson Decays Into A Highly Excited Charmed Meson Doublet
We study the heavy quark effective theory prediction for semileptonic
decays into an orbital excited -wave charmed doublet, the (, )
states (, ), at the leading order of heavy quark expansion.
The corresponding universal form factor is estimated by using the QCD sum rule
method. The decay rates we predict are and . The branching ratios are
and
, respectively.Comment: 6 pages,2 figure
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