1,423 research outputs found
Realizing orders as group rings
An order is a commutative ring that as an abelian group is finitely generated
and free. A commutative ring is reduced if it has no non-zero nilpotent
elements. In this paper we use a new tool, namely, the fact that every reduced
order has a universal grading, to answer questions about realizing orders as
group rings. In particular, we address the Isomorphism Problem for group rings
in the case where the ring is a reduced order. We prove that any non-zero
reduced order can be written as a group ring in a unique ``maximal'' way,
up to isomorphism. More precisely, there exist a ring and a finite abelian
group , both uniquely determined up to isomorphism, such that
as rings, and such that if is a ring and is a group, then
as rings if and only if there is a finite abelian group such that as rings and as groups. Computing and for
given can be done by means of an algorithm that is not quite
polynomial-time. We also give a description of the automorphism group of in
terms of and
Submillimeter wavelength survey of the galactic plane from l = -5 deg to l = +62 deg: Structure and energetics of the inner disk
Results from a large scale survey of the first quadrant of the Milky Way galactic plane at wavelengths of 150, 250, and 300 microns with a 10x10 arcmin beam are presented. The emission detected in the survey arises from compact sources, most of which are identified with known peaks of 5 GHz and/or CO emission, and from an underlying diffuse background with a typical angular width of approximately 0.9 deg (FWHM) which accounts for most of the emission. A total of 80 prominent discrete sources were identified and characterized, of which about half were not previously reported at far infrared wavelengths. The total infrared luminosity within the solar circle is approximately 1 to 2x10 to the 10th power L sub 0, and is probably emitted by dust that resides in molecular clouds
The Spectrum of Integrated Millimeter Flux of the Magellanic Clouds and 30-Doradus from TopHat and DIRBE Data
We present measurements of the integrated flux relative to the local
background of the Large and Small Magellanic Clouds and the region 30-Doradus
(the Tarantula Nebula) in the LMC in four frequency bands centered at 245, 400,
460, and 630 GHz, based on observations made with the TopHat telescope. We
combine these observations with the corresponding measurements for the DIRBE
bands 8, 9, and 10 to cover the frequency range 245 - 3000 GHz (100 - 1220
micrometers) for these objects. We present spectra for all three objects and
fit these spectra to a single-component greybody emission model and report
best-fit dust temperatures, optical depths, and emissivity power-law indices,
and we compare these results with other measurements in these regions and
elsewhere. Using published dust grain opacities, we estimate the mass of the
measured dust component in the three regions.Comment: 41 pages, 4 figures. Accepted for publication in Astrophysical
Journa
A Near-Infrared Spectrometer to Measure Zodiacal Light Absorption Spectrum
We have developed a high throughput infrared spectrometer for zodiacal light fraunhofer lines measurements. The instrument is based on a cryogenic dual silicon Fabry-Perot etalon which is designed to achieve high signal to noise Fraunhofer line profile measurements. Very large aperture silicon Fabry-Perot etalons and fast camera optics make these measurements possible. The results of the absorption line profile measurements will provide a model free measure of the zodiacal Light intensity in the near infrared. The knowledge of the zodiacal light brightness is crucial for accurate subtraction of zodiacal light foreground for accurate measure of the extragalactic background light after the subtraction of zodiacal light foreground. We present the final design of the instrument and the first results of its performance
Random Walks on a Fluctuating Lattice: A Renormalization Group Approach Applied in One Dimension
We study the problem of a random walk on a lattice in which bonds connecting
nearest neighbor sites open and close randomly in time, a situation often
encountered in fluctuating media. We present a simple renormalization group
technique to solve for the effective diffusive behavior at long times. For
one-dimensional lattices we obtain better quantitative agreement with
simulation data than earlier effective medium results. Our technique works in
principle in any dimension, although the amount of computation required rises
with dimensionality of the lattice.Comment: PostScript file including 2 figures, total 15 pages, 8 other figures
obtainable by mail from D.L. Stei
A characteristic lengthscale causes anomalous size effects and boundary programmability in mechanical metamaterials
The architecture of mechanical metamaterialsis designed to harness geometry,
non-linearity and topology to obtain advanced functionalities such as shape
morphing, programmability and one-way propagation. While a purely geometric
framework successfully captures the physics of small systems under idealized
conditions, large systems or heterogeneous driving conditions remain
essentially unexplored. Here we uncover strong anomalies in the mechanics of a
broad class of metamaterials, such as auxetics, shape-changers or topological
insulators: a non-monotonic variation of their stiffness with system size, and
the ability of textured boundaries to completely alter their properties. These
striking features stem from the competition between rotation-based
deformations---relevant for small systems---and ordinary elasticity, and are
controlled by a characteristic length scale which is entirely tunable by the
architectural details. Our study provides new vistas for designing, controlling
and programming the mechanics of metamaterials in the thermodynamic limit.Comment: Main text has 4 pages, 4 figures + Methods and Supplementary
Informatio
The COBE Diffuse Infrared Background Experiment Search for the Cosmic Infrared Background: I. Limits and Detections
The DIRBE on the COBE spacecraft was designed primarily to conduct systematic
search for an isotropic CIB in ten photometric bands from 1.25 to 240 microns.
The results of that search are presented here. Conservative limits on the CIB
are obtained from the minimum observed brightness in all-sky maps at each
wavelength, with the faintest limits in the DIRBE spectral range being at 3.5
microns (\nu I_\nu < 64 nW/m^2/sr, 95% CL) and at 240 microns (\nu I_\nu < 28
nW/m^2/sr, 95% CL). The bright foregrounds from interplanetary dust scattering
and emission, stars, and interstellar dust emission are the principal
impediments to the DIRBE measurements of the CIB. These foregrounds have been
modeled and removed from the sky maps. Assessment of the random and systematic
uncertainties in the residuals and tests for isotropy show that only the 140
and 240 microns data provide candidate detections of the CIB. The residuals and
their uncertainties provide CIB upper limits more restrictive than the dark sky
limits at wavelengths from 1.25 to 100 microns. No plausible solar system or
Galactic source of the observed 140 and 240 microns residuals can be
identified, leading to the conclusion that the CIB has been detected at levels
of \nu I_\nu = 25+-7 and 14+-3 nW/m^2/sr at 140 and 240 microns respectively.
The integrated energy from 140 to 240 microns, 10.3 nW/m^2/sr, is about twice
the integrated optical light from the galaxies in the Hubble Deep Field,
suggesting that star formation might have been heavily enshrouded by dust at
high redshift. The detections and upper limits reported here provide new
constraints on models of the history of energy-releasing processes and dust
production since the decoupling of the cosmic microwave background from matter.Comment: 26 pages and 5 figures, accepted for publication in the Astrophyical
Journa
The COBE Diffuse Infrared Background Experiment Search for the Cosmic Infrared Background: IV. Cosmological Implications
In this paper we examine the cosmological constraints of the recent DIRBE and
FIRAS detection of the extragalactic background light between 125-5000 microns
on the metal and star formation histories of the universe.Comment: 38 pages and 9 figures. Accepted for publications in The
Astrophysical Journa
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Ulcerative colitis-risk loci on chromosomes 1p36 and 12q15 found by genome-wide association study.
Ulcerative colitis is a chronic inflammatory disease of the colon that presents as diarrhea and gastrointestinal bleeding. We performed a genome-wide association study using DNA samples from 1,052 individuals with ulcerative colitis and preexisting data from 2,571 controls, all of European ancestry. In an analysis that controlled for gender and population structure, ulcerative colitis loci attaining genome-wide significance and subsequent replication in two independent populations were identified on chromosomes 1p36 (rs6426833, combined P = 5.1 x 10(-13), combined odds ratio OR = 0.73) and 12q15 (rs1558744, combined P = 2.5 x 10(-12), combined OR = 1.35). In addition, combined genome-wide significant evidence for association was found in a region spanning BTNL2 to HLA-DQB1 on chromosome 6p21 (rs2395185, combined P = 1.0 x 10(-16), combined OR = 0.66) and at the IL23R locus on chromosome 1p31 (rs11209026, combined P = 1.3 x 10(-8), combined OR = 0.56; rs10889677, combined P = 1.3 x 10(-8), combined OR = 1.29)
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