4,215 research outputs found
An Optimal Dimensionality Sampling Scheme on the Sphere for Antipodal Signals In Diffusion Magnetic Resonance Imaging
We propose a sampling scheme on the sphere and develop a corresponding
spherical harmonic transform (SHT) for the accurate reconstruction of the
diffusion signal in diffusion magnetic resonance imaging (dMRI). By exploiting
the antipodal symmetry, we design a sampling scheme that requires the optimal
number of samples on the sphere, equal to the degrees of freedom required to
represent the antipodally symmetric band-limited diffusion signal in the
spectral (spherical harmonic) domain. Compared with existing sampling schemes
on the sphere that allow for the accurate reconstruction of the diffusion
signal, the proposed sampling scheme reduces the number of samples required by
a factor of two or more. We analyse the numerical accuracy of the proposed SHT
and show through experiments that the proposed sampling allows for the accurate
and rotationally invariant computation of the SHT to near machine precision
accuracy.Comment: Will be published in the proceedings of the International Conference
Acoustics, Speech and Signal Processing 2015 (ICASSP'2015
Associated primes of graded components of local cohomology modules
The i-th local cohomology module of a finitely generated graded module M over a standard positively graded commutative Noetherian ring R with respect to the irrelevant ideal R+, is itself graded; all its graded components are finitely generated modules over R-0, the component of R of degree 0. It is known that the n-th component H-R+(i) (M)(n) of this local cohomology module H-R+(i) (M) is zero for all nmuch greater than0. This paper is concerned with the asymptotic behaviour of Ass(R0)(H-R+(i) (M)(n)) as n--> -infinity.
The smallest i for which such study is interesting is the finiteness dimension f of M relative to R+, defined as the least integer j for which H-R+(j) (M) is not finitely generated. Brodmann and Hellus have shown that AssR(0)(H-R+(f) (M)(n)) is constant for all nmuch less than0 ( that is in their terminology AssR(0)(H-R+(f) (M)(n)) is asymptotically stable for n--> -infinity). The first main aim of this paper is to identify the ultimate constant value ( under the mild assumption that R is a homomorphic image of a regular ring) : our answer is precisely the set of contractions to R-0 of certain relevant primes of R whose existence is confirmed by Grothendieck's Finiteness Theorem for local cohomology.
Brodmann and Hellus raised various questions about such asymptotic behaviour when i>f. They noted that Singh's study of a particular example ( in which f=2) shows that AssR(0)(H-R+(3) (R)(n)) need not be asymptotically stable for n--> -infinity. The second main aim of this paper is to determine, for Singh's example, AssR(0)(H-R+(3) (R)(n)) quite precisely for every integer n and, thereby answer one of the questions raised by Brodmann and Hellus
Luminosity Distributions within Rich Clusters - II: Demonstration and Verification via Simulation
We present detailed simulations of long exposure CCD images. The simulations
are used to explore the validity of the statistical method for reconstructing
the luminosity distribution of galaxies within a rich cluster i.e. by the
subtraction of field number-counts from those of a sight-line through the
cluster. In particular we use the simulations to establish the reliability of
our observational data presented in Paper 3. Based on our intended CCD
field-of-view (6.5 by 6.5 arcmins) and a 1-sigma detection limit of 26 mags per
sq arcsecond, we conclude that the luminosity distribution can be robustly
determined over a wide range of absolute magnitude (-23 < M_{R} < -16)
provided:
(a) the cluster has an Abell richness 1.5 or greater,
(b) the cluster's redshift lies in the range 0.1 < z < 0.3,
(c) the seeing is better than FWHM 1.25'' and
(d) the photometric zero points are accurate to within Delta m = \pm 0.12.
If these conditions are not met then the recovered luminosity distribution is
unreliable and potentially grossly miss-leading. Finally although the method
clearly has limitations, within these limitations the technique represents an
extremely promising probe of galaxy evolution and environmental dependencies.Comment: 24 pages, 8 figures accepted for publication in MNRAS also available
from http://star-www.st-and.ac.uk/~spd3/bib.htm
Creating Online Worlds: The Future Of Student Work
Traditional schoolwork is out of step with twenty-first century life and work. In the future, student work will focus on creating interactive online worlds instead of on writing paper-print essays
Supplying the Nuclear Arsenal
Originally published in 1996. Although the history of commercial-power nuclear reactors is well known, the story of the government reactors that produce weapons-grade plutonium and tritium has been shrouded in secrecy. Supplying the Nuclear Arsenal looks at the origin and development of these production reactors, Rodney Carlisle and Joan Zenzen describe a fifty-year government effort no less complex, expensive, and technologically demanding than the Polaris or Apollo programs—yet one about which most Americans know virtually nothing. Carlisle and Zenzen describe the evolution of the early reactors, the atomic weapons establishment that surrounded them, and the sometimes bitter struggles between business and political constituencies for their share of "nuclear pork." They show how, since the 1980s, aging production reactors have increased the risk of radioactive contamination of the atmosphere and water table. And they describe how the Department of Energy mounted a massive effort to find the right design for a new generation of reactors, only to abandon that effort with the end of the Cold War. Today, all American production reactors remain closed.Due to short half-life, the nation's supply of tritium, crucial to modern weapons, is rapidly dwindling. As countries like Iraq and North Korea threaten to join the nuclear club, the authors contend, the United States needs to revitalize tritium production capacity in order to maintain a viable nuclear deterrent. Meanwhile, as slowly decaying artifacts of the Cold War, the closed production reactors at Hanford, Washington, and Savannah River, South Carolina, loom ominously over the landscape
An Optimal Dimensionality Multi-shell Sampling Scheme with Accurate and Efficient Transforms for Diffusion MRI
This paper proposes a multi-shell sampling scheme and corresponding
transforms for the accurate reconstruction of the diffusion signal in diffusion
MRI by expansion in the spherical polar Fourier (SPF) basis. The sampling
scheme uses an optimal number of samples, equal to the degrees of freedom of
the band-limited diffusion signal in the SPF domain, and allows for
computationally efficient reconstruction. We use synthetic data sets to
demonstrate that the proposed scheme allows for greater reconstruction accuracy
of the diffusion signal than the multi-shell sampling schemes obtained using
the generalised electrostatic energy minimisation (gEEM) method used in the
Human Connectome Project. We also demonstrate that the proposed sampling scheme
allows for increased angular discrimination and improved rotational invariance
of reconstruction accuracy than the gEEM schemes.Comment: 4 pages, 4 figures presented at ISBI 201
The Future Of Online Courses: How To Win The Marketing War
Many colleges are looking to online education to improve their revenue stream. Competition for online students is escalating. Designing online courses like video games can help win the marketing war
- …