9,608 research outputs found
Information Distance: New Developments
In pattern recognition, learning, and data mining one obtains information
from information-carrying objects. This involves an objective definition of the
information in a single object, the information to go from one object to
another object in a pair of objects, the information to go from one object to
any other object in a multiple of objects, and the shared information between
objects. This is called "information distance." We survey a selection of new
developments in information distance.Comment: 4 pages, Latex; Series of Publications C, Report C-2011-45,
Department of Computer Science, University of Helsinki, pp. 71-7
Nonlinear evolution of the plasma beatwave: Compressing the laser beatnotes via electromagnetic cascading
The near-resonant beatwave excitation of an electron plasma wave (EPW) can be
employed for generating the trains of few-femtosecond electromagnetic (EM)
pulses in rarefied plasmas. The EPW produces a co-moving index grating that
induces a laser phase modulation at the difference frequency. The bandwidth of
the phase-modulated laser is proportional to the product of the plasma length,
laser wavelength, and amplitude of the electron density perturbation. The laser
spectrum is composed of a cascade of red and blue sidebands shifted by integer
multiples of the beat frequency. When the beat frequency is lower than the
electron plasma frequency, the red-shifted spectral components are advanced in
time with respect to the blue-shifted ones near the center of each laser
beatnote. The group velocity dispersion of plasma compresses so chirped
beatnotes to a few-laser-cycle duration thus creating a train of sharp EM
spikes with the beat periodicity. Depending on the plasma and laser parameters,
chirping and compression can be implemented either concurrently in the same, or
sequentially in different plasmas. Evolution of the laser beatwave end electron
density perturbations is described in time and one spatial dimension in a
weakly relativistic approximation. Using the compression effect, we demonstrate
that the relativistic bi-stability regime of the EPW excitation [G. Shvets,
Phys. Rev. Lett. 93, 195004 (2004)] can be achieved with the initially
sub-threshold beatwave pulse.Comment: 13 pages, 11 figures, submitted to Physical Review
Evaluation of registration, compression and classification algorithms. Volume 1: Results
The registration, compression, and classification algorithms were selected on the basis that such a group would include most of the different and commonly used approaches. The results of the investigation indicate clearcut, cost effective choices for registering, compressing, and classifying multispectral imagery
Dynamics of Primordial Black Hole Formation
We present a numerical investigation of the gravitational collapse of
horizon-size density fluctuations to primordial black holes (PBHs) during the
radiation-dominated phase of the Early Universe. The collapse dynamics of three
different families of initial perturbation shapes, imposed at the time of
horizon crossing, is computed. The perturbation threshold for black hole
formation, needed for estimations of the cosmological PBH mass function, is
found to be rather than the generally employed
, if is defined as \Delta M/\mh, the
relative excess mass within the initial horizon volume. In order to study the
accretion onto the newly formed black holes, we use a numerical scheme that
allows us to follow the evolution for long times after formation of the event
horizon. In general, small black holes (compared to the horizon mass at the
onset of the collapse) give rise to a fluid bounce that effectively shuts off
accretion onto the black hole, while large ones do not. In both cases, the
growth of the black hole mass owing to accretion is insignificant. Furthermore,
the scaling of black hole mass with distance from the formation threshold,
known to occur in near-critical gravitational collapse, is demonstrated to
apply to primordial black hole formation.Comment: 10 pages, 8 figures, revtex style, submitted to PR
Effective Field Theory for Bound State Reflection
Elastic quantum bound-state reflection from a hard-wall boundary provides
direct information regarding the structure and compressibility of quantum bound
states. We discuss elastic quantum bound-state reflection and derive a general
theory for elastic reflection of shallow dimers from hard-wall surfaces using
effective field theory. We show that there is a small expansion parameter for
analytic calculations of the reflection scattering length. We present a
calculation up to second order in the effective Hamiltonian in one, two, and
three dimensions. We also provide numerical lattice results for all three cases
as a comparison with our effective field theory results. Finally, we provide an
analysis of the compressibility of the alpha particle confined to a cubic
lattice with vanishing Dirichlet boundaries.Comment: 43 pages, 9 figures, 16 tables, published versio
Compressive Matched-Field Processing
Source localization by matched-field processing (MFP) generally involves
solving a number of computationally intensive partial differential equations.
This paper introduces a technique that mitigates this computational workload by
"compressing" these computations. Drawing on key concepts from the recently
developed field of compressed sensing, it shows how a low-dimensional proxy for
the Green's function can be constructed by backpropagating a small set of
random receiver vectors. Then, the source can be located by performing a number
of "short" correlations between this proxy and the projection of the recorded
acoustic data in the compressed space. Numerical experiments in a Pekeris ocean
waveguide are presented which demonstrate that this compressed version of MFP
is as effective as traditional MFP even when the compression is significant.
The results are particularly promising in the broadband regime where using as
few as two random backpropagations per frequency performs almost as well as the
traditional broadband MFP, but with the added benefit of generic applicability.
That is, the computationally intensive backpropagations may be computed offline
independently from the received signals, and may be reused to locate any source
within the search grid area
Doppler Aliasing Reduction in Wide-Angle Synthetic Aperture Radar Using a Linear Frequency Modulated Random Stepped-Frequency Waveform
This research examines the theory, application, and results of using Random Stepped-Frequency (RSF) waveforms to mitigate Doppler aliasing in a wide-angle Synthetic Aperture Radar (SAR) imaging scenario. Severe Doppler aliasing typically occurs in this scenario since range extent requirements force the pulse repetition frequency to a value violating the lower bound for Doppler aliasing. Building on previous research, this work expands upon RSF waveform analysis using a Linear Frequency Modulated RSF (LFM-RSF) waveform. The RSF waveform suppresses Doppler aliasing by positioning nulls at the aliased scatterer location. Applying LFM with RSF processing theoretically provides greater frequency coverage and aliased scatterer cancellation improvement when compared to fixed frequency values. Results using the LFM-RSF waveform show images with alias mitigation performance consistent with previous non-LFM RSF results. Given a 45 dB image dynamic range and satisfying a time-bandwidth product criterion, the LFM-RSF waveform produces an image with aliased energy reduced by 99.6%. Slightly more energy reduction is possible by violating the time-bandwidth product criterion with an associated frequency overlap between subpulses. This violation leads to additional frequency coverage which enhances aliased energy reduction to 99.9%
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