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Full Dynamics (full + dynamics)

Distribution by Scientific Domains

Physics and Astronomy	50%

Selected Abstracts

A dynamic key management solution to access hierarchy

INTERNATIONAL JOURNAL OF NETWORK MANAGEMENT, Issue 6 2007
Xukai Zou
Hierarchical access control (HAC) has been a fundamental problem in computer and network systems. Since Akl and Taylor proposed the first HAC scheme based on number theory in 1983, cryptographic key management techniques for HAC have appeared as a new and promising class of solutions to the HAC problem. Many cryptographic HAC schemes have been proposed in the past two decades. One common feature associated with these schemes is that they basically limited dynamic operations at the node level. In this paper, by introducing the innovative concept of ,access polynomial' and representing a key value as the sum of two polynomials in a finite field, we propose a new key management scheme for dynamic access hierarchy. The newly proposed scheme supports full dynamics at both the node level and user level in a uniform yet efficient manner. Furthermore, the new scheme allows access hierarchy to be a random structure and can be flexibly adapted to many other access models such as ,transfer down' and ,depth-limited transfer'. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Fragmentation model of meteoroid motion, mass loss, and radiation in the atmosphere

METEORITICS & PLANETARY SCIENCE, Issue 1 2005
k CEPLECHA
We solve them numerically including two possible types of fragmentation: into large pieces and into a cluster of small fragments. We have written a Fortran code that computes the motion, ablation and light intensity of a meteoroid at chosen heights, and allows for the ablation and shape density coefficients , and K, as well as the luminous efficiency ,, to be variable with height/time. We calibrated our fragmentation model (FM) by the best fit to observational values for the motion, ablation, radiation, fragmentation and the terminal masses (recovered meteorites) for the Lost City bolide. The FM can also handle multiple and overlapping meteor flares. We separately define both the apparent and intrinsic values of ,, K, and ,. We present in this paper values of the intrinsic luminous efficiency as function of velocity, mass, and normalized air density. Detailed results from the successful application of the FM to the Lost City, Innisfree, and Benesov bolides are also presented. Results of applying the FM to 15 bolides with very precise observational data are presented in a survey mode (Table 7). Standard deviations of applying our FM to all these events correspond to the precision of the observed values. Typical values of the intrinsic ablation coefficient are low, mostly in the range from 0.004 to 0.008 s2 km,2, and do not depend on the bolide type. The apparent ablation coefficients reflect the process of fragmentation. The bolide types indicate severity of the fragmentation process. The large differences of the "dynamic" and "photometric" mass from numerous earlier studies are completely explained by our FM. The fragmentation processes cannot be modeled simply by large values of the apparent ablation coefficient and of the apparent luminous efficiency. Moreover, our new FM can also well explain the radiation and full dynamics of very fast meteoroids at heights from 200 km to 130 km. [source]

Exciton relaxation in bulk wurtzite GaN: the role of piezoelectric interaction

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 3 2003
G. Kokolakis
Abstract A theoretical study on the relaxation of coupled free carriers and excitons after non-resonant optical excitation in bulk wurtzite GaN is presented. In particular the effect of the acoustic piezoelectric scattering is taken under consideration, and the respective rates have been calculated, including screening effects. Results show that the piezo-acoustic rates are bigger in the wurtzite phase of GaN with respect to the cubic phase, and they are really sensitive to the background doping of the sample. Simulations of the full dynamics of the system are performed by using an Ensemble Monte Carlo method under which all the relevant scattering mechanisms are included. The set of semiclassical Boltzmann equations for electron and hole populations is complemented by an additional equation for the exciton distribution and is coupled by reaction terms describing phonon-mediated exciton binding and dissociation. The temporal evolution is studied in the short range time (100 ps) after photo-excitation. It shows that a high background doping prevents the electrons to relax toward low energy states. [source]

Economic significance of risk premiums in the S&P 500 option market

THE JOURNAL OF FUTURES MARKETS, Issue 12 2002
R. Brian Balyeat
Option pricing is complicated by the theoretical existence of risk premiums. This article utilizes a testable methodology to extract the pricing impact resulting from these risk premiums. First, option prices (based on the full dynamics of the underlying) are computed under the assumption that these risk premiums are not priced. The pricing methodology is independent of any particular option-pricing model or distributional assumptions on the return process for the underlying. The difference between the actual market prices and these "no-premium base case" prices reflects the effect of risk premiums. For at-the-money, 13-week S&P 500 options trading from 1989 until 1993, the effect of risk premiums is statistically significant and averages slightly over 20% (in units of Black,Scholes implied volatility). A simple delta-hedging strategy is used to demonstrate the economic significance of risk premiums, as the trading strategy provides enough profit to absorb a crash similar in magnitude to the 1987 crash once every 6.20 years. © 2002 Wiley Periodicals, Inc. Jrl Fut Mark 22:1147,1178, 2002 [source]