301 research outputs found
A dynamical description of neutron star crusts
Neutron Stars are natural laboratories where fundamental properties of matter
under extreme conditions can be explored. Modern nuclear physics input as well
as many-body theories are valuable tools which may allow us to improve our
understanding of the physics of those compact objects.
In this work the occurrence of exotic structures in the outermost layers of
neutron stars is investigated within the framework of a microscopic model. In
this approach the nucleonic dynamics is described by a time-dependent mean
field approach at around zero temperature. Starting from an initial crystalline
lattice of nuclei at subnuclear densities the system evolves toward a manifold
of self-organized structures with different shapes and similar energies. These
structures are studied in terms of a phase diagram in density and the
corresponding sensitivity to the isospin-dependent part of the equation of
state and to the isotopic composition is investigated.Comment: 8 pages, 5 figures, conference NN201
A matrix pencil approach to the existence of compactly supported reconstruction functions in average sampling
The aim of this work is to solve a question raised for average sampling in shift-invariant spaces by using the well-known matrix pencil theory. In many common situations in sampling theory, the available data are samples of some convolution operator acting on the function itself: this leads to the problem of average sampling, also known as generalized sampling. In this paper we deal with the existence of a sampling formula involving these samples and having reconstruction functions with compact support. Thus, low computational complexity is involved and truncation errors are avoided. In practice, it is accomplished by means of a FIR filter bank. An answer is given in the light of the generalized sampling theory by using the oversampling technique: more samples than strictly necessary are used. The original problem reduces to finding a polynomial left inverse of a polynomial matrix intimately related to the sampling problem which, for a suitable choice of the sampling period, becomes a matrix pencil. This matrix pencil approach allows us to obtain a practical method for computing the compactly supported reconstruction functions for the important case where the oversampling rate is minimum. Moreover, the optimality of the obtained solution is established
An Optimized Spline-Based Registration of a 3D CT to a Set of C-Arm Images
We have developed an algorithm for the rigid-body registration of
a CT volume to a set of C-arm images.
The algorithm uses a gradient-based iterative minimization of a least-squares measure
of dissimilarity between the C-arm images and projections of the
CT volume. To compute projections, we use a novel method for fast
integration of the volume along rays. To improve robustness and
speed, we take advantage of a coarse-to-fine processing of the
volume/image pyramids. To compute the projections of the volume,
the gradient of the dissimilarity measure, and the multiresolution
data pyramids, we use a continuous image/volume model based on
cubic B-splines, which ensures a high interpolation accuracy and a
gradient of the dissimilarity measure that is well defined
everywhere. We show the performance of our algorithm on a human
spine phantom, where the true alignment is determined using a set
of fiducial markers
Operation of the DC Current Transformer intensity monitors at FNAL during Run II
Circulating beam intensity measurements at FNAL are provided by five DC current transformers (DCCT), one per machine. With the exception of the DCCT in the Recycler, all DCCT systems were designed and built at FNAL. This paper presents an overview of both DCCT systems, including the sensor, the electronics, and the front-end instrumentation software, as well as their performance during Run II
Justification of the coupled-mode approximation for a nonlinear elliptic problem with a periodic potential
Coupled-mode systems are used in physical literature to simplify the
nonlinear Maxwell and Gross-Pitaevskii equations with a small periodic
potential and to approximate localized solutions called gap solitons by
analytical expressions involving hyperbolic functions. We justify the use of
the one-dimensional stationary coupled-mode system for a relevant elliptic
problem by employing the method of Lyapunov--Schmidt reductions in Fourier
space. In particular, existence of periodic/anti-periodic and decaying
solutions is proved and the error terms are controlled in suitable norms. The
use of multi-dimensional stationary coupled-mode systems is justified for
analysis of bifurcations of periodic/anti-periodic solutions in a small
multi-dimensional periodic potential.Comment: 18 pages, no figure
Fractional Operators, Dirichlet Averages, and Splines
Fractional differential and integral operators, Dirichlet averages, and
splines of complex order are three seemingly distinct mathematical subject
areas addressing different questions and employing different methodologies. It
is the purpose of this paper to show that there are deep and interesting
relationships between these three areas. First a brief introduction to
fractional differential and integral operators defined on Lizorkin spaces is
presented and some of their main properties exhibited. This particular approach
has the advantage that several definitions of fractional derivatives and
integrals coincide. We then introduce Dirichlet averages and extend their
definition to an infinite-dimensional setting that is needed to exhibit the
relationships to splines of complex order. Finally, we focus on splines of
complex order and, in particular, on cardinal B-splines of complex order. The
fundamental connections to fractional derivatives and integrals as well as
Dirichlet averages are presented
Wavelet analysis of epileptic spikes
Interictal spikes and sharp waves in human EEG are characteristic signatures
of epilepsy. These potentials originate as a result of synchronous,
pathological discharge of many neurons. The reliable detection of such
potentials has been the long standing problem in EEG analysis, especially after
long-term monitoring became common in investigation of epileptic patients. The
traditional definition of a spike is based on its amplitude, duration,
sharpness, and emergence from its background. However, spike detection systems
built solely around this definition are not reliable due to the presence of
numerous transients and artifacts. We use wavelet transform to analyze the
properties of EEG manifestations of epilepsy. We demonstrate that the behavior
of wavelet transform of epileptic spikes across scales can constitute the
foundation of a relatively simple yet effective detection algorithm.Comment: 4 pages, 3 figure
EPW: A program for calculating the electron-phonon coupling using maximally localized Wannier functions
EPW (Electron-Phonon coupling using Wannier functions) is a program written
in FORTRAN90 for calculating the electron-phonon coupling in periodic systems
using density-functional perturbation theory and maximally-localized Wannier
functions. EPW can calculate electron-phonon interaction self-energies,
electron-phonon spectral functions, and total as well as mode-resolved
electron-phonon coupling strengths. The calculation of the electron-phonon
coupling requires a very accurate sampling of electron-phonon scattering
processes throughout the Brillouin zone, hence reliable calculations can be
prohibitively time-consuming. EPW combines the Kohn-Sham electronic eigenstates
and the vibrational eigenmodes provided by the Quantum-ESPRESSO package [1]
with the maximally localized Wannier functions provided by the wannier90
package [2] in order to generate electron-phonon matrix elements on arbitrarily
dense Brillouin zone grids using a generalized Fourier interpolation. This
feature of EPW leads to fast and accurate calculations of the electron-phonon
coupling, and enables the study of the electron-phonon coupling in large and
complex systems.Comment: 6 figure
Model-Based Estimation of Three-Dimensional Stiffness Parameters in Photonic-Force Microscopy
We propose a system to characterize the 3-D diffusion properties of the probing bead trapped by a photonic-force microscope. We follow a model-based approach, where the model of the dynamics of the bead is given by the Langevin equation. Our procedure combines software and analog hardware to measure the corresponding stiffness matrix. We are able to estimate all its elements in real time, including off-diagonal terms. To achieve our goal, we have built a simple analog computer that performs a continuous preprocessing of the data, which can be subsequently digitized at a much lower rate than is otherwise required. We also provide an effective numerical algorithm for compensating the correlation bias introduced by a quadrant photodiode detector in the microscope. We validate our approach using simulated data and show that our bias-compensation scheme effectively improves the accuracy of the system. Moreover, we perform experiments with the real system and demonstrate real-time capabilities. Finally, we suggest a simple adjunction that would allow one to determine the mass matrix as well
Effect of aging on elastin functionality in human cerebral arteries
Aging affects elastin, a key component of the arterial wall integrity and functionality. Elastin degradation in cerebral vessels is associated with cerebrovascular disease. The goal of this study is to assess the biomechanical properties of human cerebral arteries, their composition, and their geometry, with particular focus on the functional alteration of elastin attributable to aging
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