169,397 research outputs found
Non-fragile H∞ control with randomly occurring gain variations, distributed delays and channel fadings
This study is concerned with the non-fragile H∞ control problem for a class of discrete-time systems subject to randomly occurring gain variations (ROGVs), channel fadings and infinite-distributed delays. A new stochastic phenomenon (ROGVs), which is governed by a sequence of random variables with a certain probabilistic distribution, is put forward to better reflect the reality of the randomly occurring fluctuation of controller gains implemented in networked environments. A modified stochastic Rice fading model is then exploited to account for both channel fadings and random time-delays in a unified representation. The channel coefficients are a set of mutually independent random variables which abide by any (not necessarily Gaussian) probability density function on [0, 1]. Attention is focused on the analysis and design of a non-fragile H∞ outputfeedback controller such that the closed-loop control system is stochastically stable with a prescribed H∞ performance. Through intensive stochastic analysis, sufficient conditions are established for the desired stochastic stability and H∞ disturbance attenuation, and the addressed non-fragile control problem is then recast as a convex optimisation problem solvable via the semidefinite programme method. An example is finally provided to demonstrate the effectiveness of the proposed design method
Chandra detection of diffuse hot gas in and around the M31 bulge
We report the detection of diffuse hot gas in M31, using archival Chandra
observations which allow us to map out a 30' by 30' field (covering a
galactocentric radius up to 4.5 kpc) and to detect sources in the galaxy down
to a 0.5-8 keV luminosity limit of ~10^35 ergs/s. We estimate the remaining
stellar contribution from fainter X-ray sources (primarily cataclysmic
variables and coronally active binaries), assuming that they spatially follow
the stellar distribution. Indeed, the near-IR K-band light of the galaxy
closely traces the 2-8 keV unresolved X-rays, indicating a collective stellar
X-ray emissivity consistent with those determined for the Galactic ridge and
M32, whereas the amount of the 0.5-2 keV unresolved emission is significantly
greater than the expected stellar contribution, especially within a
galactocentric radius of ~2 kpc. Morphologically, this soft X-ray excess
appears substantially rounder than the bulge as seen in K-band and is elongated
approximately along the minor-axis at large radii. The excess thus most likely
represents the emission of diffuse hot gas in and around the galactic bulge.
Furthermore, the near side of the M31 disk casts an apparent shadow against the
soft X-ray excess, indicating that the hot gas extends to at least 2.5 kpc from
the galactic plane. We briefly discuss the implications of these results on the
energy balance in the M31 bulge and on understanding the large-scale soft X-ray
enhancement observed toward the inner region of our own Galaxy.Comment: 10 pages, 2 figures, accepted by ApJ
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H∞ fault estimation with randomly occurring uncertainties, quantization effects and successive packet dropouts: The finite-horizon case
In this paper, the finite-horizon H∞ fault estimation problem is investigated for a class of uncertain nonlinear time-varying systems subject to multiple stochastic delays. The randomly occurring uncertainties (ROUs) enter into the system due to the random fluctuations of network conditions. The measured output is quantized by a logarithmic quantizer before being transmitted to the fault estimator. Also, successive packet dropouts (SPDs) happen when the quantized signals are transmitted through an unreliable network medium. Three mutually independent sets of Bernoulli-distributed white sequences are introduced to govern the multiple stochastic delays, ROUs and SPDs. By employing the stochastic analysis approach, some sufficient conditions are established for the desired finite-horizon fault estimator to achieve the specified H∞ performance. The time-varying parameters of the fault estimator are obtained by solving a set of recursive linear matrix inequalities. Finally, an illustrative numerical example is provided to show the effectiveness of the proposed fault estimation approach
Annihilation Type Radiative Decays of Meson in Perturbative QCD Approach
With the perturbative QCD approach based on factorization, we study the
pure annihilation type radiative decays and . We find that the branching ratio of is
, which is too small to be measured
in the current factories of BaBar and Belle. The branching ratio of is , which is just
at the corner of being observable in the factories. A larger branching
ratio is also predicted.
These decay modes will help us testing the standard model and searching for new
physics signals.Comment: 4 pages, revtex, with 1 eps figur
The IT performance evaluation in the construction industry
To date there has been limited published work in
the construction management and engineering
literature that has provided empirical evidence to
demonstrate that IT can improve organizational
performance. Without an explicit understanding
about how IT can be effectively used to improve
organizational performance, its justification will
remain to be weak for managers. To ensure the
continuous increase in IT based applications in the
construction industry, sufficient evidence has to be
provided for management in various professions of
the construction industry to evaluate, allocate and
utilize appropriate IT systems. In an attempt to
explore the relationship between IT and
productivity, an empirical investigation of 60
Professional Consulting Firms (PCF) from the
Hong Kong construction industry was undertaken.
A model for determining the organizational
productivity of IT is proposed, and the
methodology used to test the model is described.
The findings are analyzed and a cross-profession
comparison of the results indicated the differences
in the use of IT. The research findings are discussed
with similarities being drawn. The limitations of the
research are then presented and discussed. The
implications of the findings and conclusions then
fully presented
Fast ground-state cooling of mechanical resonator with time-dependent optical cavities
We propose a feasible scheme to cool down a mechanical resonator (MR) in a
three-mirror cavity optomechanical system with controllable external optical
drives. Under the Born-Oppenheimer (BO) approximation, the whole dynamics of
the mechanical resonator and cavities is reduced to that of a time-dependent
harmonic oscillator, whose effective frequency can be controlled through the
optical driving fields. The fast cooling of the MR can be realized by
controlling the amplitude of the optical drives. Significantly, we further show
that the ground-state cooling may be achieved via the three-mirror cavity
optomechanical system without the resolved sideband condition.Comment: Some references including our previous works on cooling of mechanical
resonators are added, and some typos are corrected in this new version.
Comments are welcom
Magneto-Centrifugal Launching of Jets from Accretion Disks. I: Cold Axisymmetric Flows
The magneto-centrifugal model for jet formation is studied by time-dependent
simulations reaching steady state in a cold gas with negligible fluid pressure,
in an axisymmetric geometry, using a modification of the Zeus3D code adapted to
parallel computers. The number of boundary conditions imposed at the coronal
base takes into account the existence of the fast and Alfvenic critical
surfaces, avoiding over-determination of the flow. The size and shape of the
computational box is chosen to include these critical surfaces, reducing the
influence of the outer boundary conditions. As there is a region, near the
origin, where the inclination of field lines to the axis is too small to drive
a centrifugal wind, we inject a thin, axial jet, expected to form
electromagnetically near black holes. Acceleration and collimation appear for
wide generic conditions. A reference run is shown in detail, with a wind
leaving the computational volume in the axial direction with a poloidal
velocity equal to 4 times the poloidal Alfven speed, collimated inside 11
degrees. Finally, the critical surfaces, fieldlines, thrust, energy, torque and
mass discharge of the outgoing wind are shown for simulations with various
profiles of mass and magnetic flux at the base of the corona.Comment: 27 pages, including 10 figures and 2 tables. To appear in ApJ (Dec
1999). Revised version clarifies the abstract, section 3.2.4, conclusions and
appendix, adds a simulation to section 4.2, and updates the reference
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