1,011 research outputs found
On the existence of star products on quotient spaces of linear Hamiltonian torus actions
We discuss BFV deformation quantization of singular symplectic quotient
spaces in the special case of linear Hamiltonian torus actions. In particular,
we show that the Koszul complex on the moment map of an effective linear
Hamiltonian torus action is acyclic. We rephrase the nonpositivity condition of
Arms, Gotay and Jennings for linear Hamiltonian torus actions. It follows that
reduced spaces of such actions admit continuous star products.Comment: 9 pages, 4 figures, uses psfra
The pseudogap state in superconductors: Extended Hartree approach to time-dependent Ginzburg-Landau Theory
It is well known that conventional pairing fluctuation theory at the Hartree
level leads to a normal state pseudogap in the fermionic spectrum. Our goal is
to extend this Hartree approximated scheme to arrive at a generalized mean
field theory of pseudogapped superconductors for all temperatures . While an
equivalent approach to the pseudogap has been derived elsewhere using a more
formal Green's function decoupling scheme, in this paper we re-interpret this
mean field theory and BCS theory as well, and demonstrate how they naturally
relate to ideal Bose gas condensation. Here we recast the Hartree approximated
Ginzburg-Landau self consistent equations in a T-matrix form. This recasting
makes it possible to consider arbitrarily strong attractive coupling, where
bosonic degrees of freedom appear at considerably above . The
implications for transport both above and below are discussed. Below
we find two types of contributions. Those associated with fermionic
excitations have the usual BCS functional form. That they depend on the
magnitude of the excitation gap, nevertheless, leads to rather atypical
transport properties in the strong coupling limit, where this gap (as distinct
from the order parameter) is virtually -independent. In addition, there are
bosonic terms arising from non-condensed pairs whose transport properties are
shown here to be reasonably well described by an effective time-dependent
Ginzburg-Landau theory.Comment: 14 pages, 5 figures, REVTeX4, submitted to PRB; clarification of the
diagrammatic technique added, one figure update
An HST Snapshot Survey of Proto-Planetary Nebulae Candidates: Two Types of Axisymmetric Reflection Nebulosities
We report the results from an optical imaging survey of proto-planetary
nebula candidates using the HST. We exploited the high resolving power and wide
dynamic range of HST and detected nebulosities in 21 of 27 sources. All
detected reflection nebulosities show elongation, and the nebula morphology
bifurcates depending on the degree of the central star obscuration. The
Star-Obvious Low-level-Elongated (SOLE) nebulae show a bright central star
embedded in a faint, extended nebulosity, whereas the DUst-Prominent
Longitudinally-EXtended (DUPLEX) nebulae have remarkable bipolar structure with
a completely or partially obscured central star. The intrinsic axisymmetry of
these proto-planetary nebula reflection nebulosities demonstrates that the
axisymmetry frequently found in planetary nebulae predates the proto-planetary
nebula phase, confirming previous independent results. We suggest that
axisymmetry in proto-planetary nebulae is created by an equatorially enhanced
superwind at the end of the asymptotic giant branch phase. We discuss that the
apparent morphological dichotomy is caused by a difference in the optical
thickness of the circumstellar dust/gas shell with a differing equator-to-pole
density contrast. Moreover, we show that SOLE and DUPLEX nebulae are physically
distinct types of proto-planetary nebulae, with a suggestion that higher mass
progenitor AGB stars are more likely to become DUPLEX proto-planetary nebulae.Comment: 27 pages (w/ aaspp4.sty), 6 e/ps figures, 4 tables (w/ apjpt4.sty).
Data images are available via ADIL
(http://imagelib.ncsa.uiuc.edu/document/99.TU.01) To be published in Ap
A Mid-Infrared Imaging Survey of Proto-Planetary Nebula Candidates
We present the data from a mid-infrared imaging survey of 66 proto-planetary
nebula candidates using two mid-IR cameras (MIRAC2 and Berkcam) at the NASA
Infrared Telescope Facility and the United Kingdom Infrared Telescope. The goal
of this survey is to determine the size, flux, and morphology of the mid-IR
emission regions, which sample the inner regions of the circumstellar dust
shells of proto-planetary nebulae. We imaged these proto-planetary nebulae with
narrow-band filters () at wavelengths of
notable dust features. With our typical angular resolution of 1\arcsec, we
resolve 17 sources, find 48 objects unresolved, and do not detect 1 source. For
several sources, we checked optical and infrared associations and positions of
the sources. In table format, we list the size and flux measurements for all
the detected objects and show figures of all the resolved sources. Images for
all the detected objects are available on line in FITS format from the
Astronomy Digital Image Library at the National Center for Supercomputing
Application. The proto-planetary nebula candidate sample includes, in addition
to the predominant proto-planetary nebulae, extreme asymptotic giant branch
stars, young planetary nebulae, a supergiant, and a luminous blue variable. We
find that dust shells which are cooler ( K) and brighter in the
infrared are more easily resolved. Eleven of the seventeen resolved sources are
extended and fall into one of two types of mid-IR morphological classes:
core/elliptical or toroidal. Core/elliptical structures show unresolved cores
with lower surface brightness elliptical nebulae. Toroidal structures show
limb-brightened peaks suggesting equatorial density enhancements. We argue that
core/ellipticals have denser dust shells than toroidals.Comment: 32 pages, 5 tables, 2 e/ps figures (fig3 is available through ADIL
[see text]), to be published in ApJS May 1999 issu
A hierarchical key pre-distribution scheme for fog networks
Security in fog computing is multi-faceted, and one particular challenge is establishing a secure communication channel between fog nodes and end devices. This emphasizes the importance of designing efficient and secret key distribution scheme to facilitate fog nodes and end devices to establish secure communication channels. Existing secure key distribution schemes designed for hierarchical networks may be deployable in fog computing, but they incur high computational and communication overheads and thus consume significant memory. In this paper, we propose a novel hierarchical key pre-distribution scheme based on “Residual Design” for fog networks. The proposed key distribution scheme is designed to minimize storage overhead and memory consumption, while increasing network scalability. The scheme is also designed to be secure against node capture attacks. We demonstrate that in an equal-size network, our scheme achieves around 84% improvement in terms of node storage overhead, and around 96% improvement in terms of network scalability. Our research paves the way for building an efficient key management framework for secure communication within the hierarchical network of fog nodes and end devices.
KEYWORDS: Fog Computing, Key distribution, Hierarchical Networks
Interpretation of heart rate variability via detrended fluctuation analysis and alpha-beta filter
Detrended fluctuation analysis (DFA), suitable for the analysis of
nonstationary time series, has confirmed the existence of persistent long-range
correlations in healthy heart rate variability data. In this paper, we present
the incorporation of the alpha-beta filter to DFA to determine patterns in the
power-law behaviour that can be found in these correlations. Well-known
simulated scenarios and real data involving normal and pathological
circumstances were used to evaluate this process. The results presented here
suggest the existence of evolving patterns, not always following a uniform
power-law behaviour, that cannot be described by scaling exponents estimated
using a linear procedure over two predefined ranges. Instead, the power law is
observed to have a continuous variation with segment length. We also show that
the study of these patterns, avoiding initial assumptions about the nature of
the data, may confer advantages to DFA by revealing more clearly abnormal
physiological conditions detected in congestive heart failure patients related
to the existence of dominant characteristic scales.Comment: 18 pages, 14 figure
Effect of Trends on Detrended Fluctuation Analysis
Detrended fluctuation analysis (DFA) is a scaling analysis method used to
estimate long-range power-law correlation exponents in noisy signals. Many
noisy signals in real systems display trends, so that the scaling results
obtained from the DFA method become difficult to analyze. We systematically
study the effects of three types of trends -- linear, periodic, and power-law
trends, and offer examples where these trends are likely to occur in real data.
We compare the difference between the scaling results for artificially
generated correlated noise and correlated noise with a trend, and study how
trends lead to the appearance of crossovers in the scaling behavior. We find
that crossovers result from the competition between the scaling of the noise
and the ``apparent'' scaling of the trend. We study how the characteristics of
these crossovers depend on (i) the slope of the linear trend; (ii) the
amplitude and period of the periodic trend; (iii) the amplitude and power of
the power-law trend and (iv) the length as well as the correlation properties
of the noise. Surprisingly, we find that the crossovers in the scaling of noisy
signals with trends also follow scaling laws -- i.e. long-range power-law
dependence of the position of the crossover on the parameters of the trends. We
show that the DFA result of noise with a trend can be exactly determined by the
superposition of the separate results of the DFA on the noise and on the trend,
assuming that the noise and the trend are not correlated. If this superposition
rule is not followed, this is an indication that the noise and the superimposed
trend are not independent, so that removing the trend could lead to changes in
the correlation properties of the noise.Comment: 20 pages, 16 figure
Magnetic Field Effects in the Pseudogap Phase: A Competing Energy Gap Scenario for Precursor Superconductivity
We study the sensitivity of T_c and T^* to low fields, H, within the
pseudogap state using a BCS-based approach extended to arbitrary coupling. We
find that T^* and T_c, which are of the same superconducting origin, have very
different H dependences. This is due to the pseudogap, \Delta_{pg}, which is
present at the latter, but not former temperature. Our results for the
coherence length \xi fit well with existing experiments.We predict that very
near the insulator \xi will rapidly increase.Comment: 4 pages, 4 figures, RevTe
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