15,482 research outputs found
Synthesis of Covert Actuator Attackers for Free
In this paper, we shall formulate and address a problem of covert actuator
attacker synthesis for cyber-physical systems that are modelled by
discrete-event systems. We assume the actuator attacker partially observes the
execution of the closed-loop system and is able to modify each control command
issued by the supervisor on a specified attackable subset of controllable
events. We provide straightforward but in general exponential-time reductions,
due to the use of subset construction procedure, from the covert actuator
attacker synthesis problems to the Ramadge-Wonham supervisor synthesis
problems. It then follows that it is possible to use the many techniques and
tools already developed for solving the supervisor synthesis problem to solve
the covert actuator attacker synthesis problem for free. In particular, we show
that, if the attacker cannot attack unobservable events to the supervisor, then
the reductions can be carried out in polynomial time. We also provide a brief
discussion on some other conditions under which the exponential blowup in state
size can be avoided. Finally, we show how the reduction based synthesis
procedure can be extended for the synthesis of successful covert actuator
attackers that also eavesdrop the control commands issued by the supervisor.Comment: The paper has been accepted for the journal Discrete Event Dynamic
System
Spin tunneling properties in mesoscopic magnets: effects of a magnetic field
The tunneling of a giant spin at excited levels is studied theoretically in
mesoscopic magnets with a magnetic field at an arbitrary angle in the easy
plane. Different structures of the tunneling barriers can be generated by the
magnetocrystalline anisotropy, the magnitude and the orientation of the field.
By calculating the nonvacuum instanton solution explicitly, we obtain the
tunnel splittings and the tunneling rates for different angle ranges of the
external magnetic field ( and ). The
temperature dependences of the decay rates are clearly shown for each case. It
is found that the tunneling rate and the crossover temperature depend on the
orientation of the external magnetic field. This feature can be tested with the
use of existing experimental techniques.Comment: 27 pages, 4 figures, accepted by Euro. Phys. J.
Field-dependent quantum nucleation of antiferromagnetic bubbles
The phenomenon of quantum nucleation is studied in a nanometer-scale
antiferromagnet with biaxial symmetry in the presence of a magnetic field at an
arbitrary angle. Within the instanton approach, we calculate the dependence of
the rate of quantum nucleation and the crossover temperature on the orientation
and strength of the field for bulk solids and two-dimensional films of
antiferromagnets, respectively. Our results show that the rate of quantum
nucleation and the crossover temperature from thermal-to-quantum transitions
depend on the orientation and strength of the field distinctly, which can be
tested with the use of existing experimental techniques.Comment: 21 pages, 5 figures, Final version and accepted by Eur. Phys. J
Lookahead Strategies for Sequential Monte Carlo
Based on the principles of importance sampling and resampling, sequential
Monte Carlo (SMC) encompasses a large set of powerful techniques dealing with
complex stochastic dynamic systems. Many of these systems possess strong
memory, with which future information can help sharpen the inference about the
current state. By providing theoretical justification of several existing
algorithms and introducing several new ones, we study systematically how to
construct efficient SMC algorithms to take advantage of the "future"
information without creating a substantially high computational burden. The
main idea is to allow for lookahead in the Monte Carlo process so that future
information can be utilized in weighting and generating Monte Carlo samples, or
resampling from samples of the current state.Comment: Published in at http://dx.doi.org/10.1214/12-STS401 the Statistical
Science (http://www.imstat.org/sts/) by the Institute of Mathematical
Statistics (http://www.imstat.org
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