450 research outputs found
Verification and Control of Partially Observable Probabilistic Real-Time Systems
We propose automated techniques for the verification and control of
probabilistic real-time systems that are only partially observable. To formally
model such systems, we define an extension of probabilistic timed automata in
which local states are partially visible to an observer or controller. We give
a probabilistic temporal logic that can express a range of quantitative
properties of these models, relating to the probability of an event's
occurrence or the expected value of a reward measure. We then propose
techniques to either verify that such a property holds or to synthesise a
controller for the model which makes it true. Our approach is based on an
integer discretisation of the model's dense-time behaviour and a grid-based
abstraction of the uncountable belief space induced by partial observability.
The latter is necessarily approximate since the underlying problem is
undecidable, however we show how both lower and upper bounds on numerical
results can be generated. We illustrate the effectiveness of the approach by
implementing it in the PRISM model checker and applying it to several case
studies, from the domains of computer security and task scheduling
Phases and relativity in atomic gravimetry
The phase observable measured by an atomic gravimeter built up on stimulated
Raman transitions is discussed in a fully relativistic context. It is written
in terms of laser phases which are invariant under relativistic gauge
transformations. The dephasing is the sum of light and atomic contributions
which are connected to one another through their interplay with conservation
laws at the interaction vertices. In the case of a closed geometry, a compact
form of the dephasing is written in terms of a Legendre transform of the laser
phases. These general expressions are illustrated by discussing two techniques
used for compensating the Doppler shift, one corresponding to chirped
frequencies and the other one to ramped variations.Comment: 7 pages, 1 figur
LTL Parameter Synthesis of Parametric Timed Automata
The parameter synthesis problem for parametric timed automata is undecidable
in general even for very simple reachability properties. In this paper we
introduce restrictions on parameter valuations under which the parameter
synthesis problem is decidable for LTL properties. The investigated bounded
integer parameter synthesis problem could be solved using an explicit
enumeration of all possible parameter valuations. We propose an alternative
symbolic zone-based method for this problem which results in a faster
computation. Our technique extends the ideas of the automata-based approach to
LTL model checking of timed automata. To justify the usefulness of our
approach, we provide experimental evaluation and compare our method with
explicit enumeration technique.Comment: 23 pages, extended versio
Interrupt Timed Automata: verification and expressiveness
We introduce the class of Interrupt Timed Automata (ITA), a subclass of
hybrid automata well suited to the description of timed multi-task systems with
interruptions in a single processor environment. While the reachability problem
is undecidable for hybrid automata we show that it is decidable for ITA. More
precisely we prove that the untimed language of an ITA is regular, by building
a finite automaton as a generalized class graph. We then establish that the
reachability problem for ITA is in NEXPTIME and in PTIME when the number of
clocks is fixed. To prove the first result, we define a subclass ITA- of ITA,
and show that (1) any ITA can be reduced to a language-equivalent automaton in
ITA- and (2) the reachability problem in this subclass is in NEXPTIME (without
any class graph). In the next step, we investigate the verification of real
time properties over ITA. We prove that model checking SCL, a fragment of a
timed linear time logic, is undecidable. On the other hand, we give model
checking procedures for two fragments of timed branching time logic. We also
compare the expressive power of classical timed automata and ITA and prove that
the corresponding families of accepted languages are incomparable. The result
also holds for languages accepted by controlled real-time automata (CRTA), that
extend timed automata. We finally combine ITA with CRTA, in a model which
encompasses both classes and show that the reachability problem is still
decidable. Additionally we show that the languages of ITA are neither closed
under complementation nor under intersection
Efficient Emptiness Check for Timed B\"uchi Automata (Extended version)
The B\"uchi non-emptiness problem for timed automata refers to deciding if a
given automaton has an infinite non-Zeno run satisfying the B\"uchi accepting
condition. The standard solution to this problem involves adding an auxiliary
clock to take care of the non-Zenoness. In this paper, it is shown that this
simple transformation may sometimes result in an exponential blowup. A
construction avoiding this blowup is proposed. It is also shown that in many
cases, non-Zenoness can be ascertained without extra construction. An
on-the-fly algorithm for the non-emptiness problem, using non-Zenoness
construction only when required, is proposed. Experiments carried out with a
prototype implementation of the algorithm are reported.Comment: Published in the Special Issue on Computer Aided Verification - CAV
2010; Formal Methods in System Design, 201
Teachers developing assessment for learning: impact on student achievement
While it is generally acknowledged that increased use of formative assessment (or assessment for learning) leads to higher quality learning, it is often claimed that the pressure in schools to improve the results achieved by students in externally-set tests and examinations precludes its use. This paper reports on the achievement of secondary school students who worked in classrooms where teachers made time to develop formative assessment strategies. A total of 24 teachers (2 science and 2 mathematics teachers, in each of six schools in two LEAs) were supported over a six-month period in exploring and planning their approach to formative assessment, and then, beginning in September 1999, the teachers put these plans into action with selected classes. In order to compute effect sizes, a measure of prior attainment and at least one comparison group was established for each class (typically either an equivalent class taught in the previous year by the same teacher, or a parallel class taught by another teacher). The mean effect size was 0.32
Quasi-continuous atom laser in the presence of gravity
We analyse the extraction of a coherent atomic beam from a trapped
Bose-Einstein condensate using a rf transition to a non-trapping state at T=0
K. Our quantum treatment fully takes gravity into account but neglects all
interactions in the free falling beam. We obtain an analytical expression of
the output rate and of the wave function of the extracted beam, i.e. the output
mode of the ``atom laser''. Our model reproduces satisfactorily experimental
data without any adjustable parameter.Comment: 4 pages, 2 figure
Verification and Control of Turn-Based Probabilistic Real-Time Games
Quantitative verification techniques have been developed for the formal analysis of a variety of probabilistic models, such as Markov chains, Markov decision process and their variants. They can be used to produce guarantees on quantitative aspects of system behaviour, for example safety, reliability and performance, or to help synthesise controllers that ensure such guarantees are met. We propose the model of turn-based probabilistic timed multi-player games, which incorporates probabilistic choice, real-time clocks and nondeterministic behaviour across multiple players. Building on the digital clocks approach for the simpler model of probabilistic timed automata, we show how to compute the key measures that underlie quantitative verification, namely the probability and expected cumulative price to reach a target. We illustrate this on case studies from computer security and task scheduling
Model Checking Classes of Metric LTL Properties of Object-Oriented Real-Time Maude Specifications
This paper presents a transformational approach for model checking two
important classes of metric temporal logic (MTL) properties, namely, bounded
response and minimum separation, for nonhierarchical object-oriented Real-Time
Maude specifications. We prove the correctness of our model checking
algorithms, which terminate under reasonable non-Zeno-ness assumptions when the
reachable state space is finite. These new model checking features have been
integrated into Real-Time Maude, and are used to analyze a network of medical
devices and a 4-way traffic intersection system.Comment: In Proceedings RTRTS 2010, arXiv:1009.398
Kinetics of macroion coagulation induced by multivalent counterions
Due to the strong correlations between multivalent counterions condensed on a
macroion, the net macroion charge changes sign at some critical counterion
concentration. This effect is known as the charge inversion. Near this critical
concentration the macroion net charge is small. Therefore, short range
attractive forces between macroions dominate Coulomb repulsion and lead to
their coagulation. The kinetics of macroion coagulation in this range of
counterion concentrations is studied. We calculate the Coulomb barrier between
two approaching like charged macroions at a given counterion concentration. Two
different macroion shapes (spherical and rod-like) are considered. A new
"self-regulated" regime of coagulation is found. As the size of aggregates
increases, their charge and Coulomb barrier also grow and diminish the sticking
probability of aggregates. This leads to a slow, logarithmic increase of the
aggregate size with time.Comment: Some formulas correcte
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