681 research outputs found
Better abstractions for timed automata
We consider the reachability problem for timed automata. A standard solution
to this problem involves computing a search tree whose nodes are abstractions
of zones. These abstractions preserve underlying simulation relations on the
state space of the automaton. For both effectiveness and efficiency reasons,
they are parametrized by the maximal lower and upper bounds (LU-bounds)
occurring in the guards of the automaton. We consider the aLU abstraction
defined by Behrmann et al. Since this abstraction can potentially yield
non-convex sets, it has not been used in implementations. We prove that aLU
abstraction is the biggest abstraction with respect to LU-bounds that is sound
and complete for reachability. We also provide an efficient technique to use
the aLU abstraction to solve the reachability problem.Comment: Extended version of LICS 2012 paper (conference paper till v6). in
Information and Computation, available online 27 July 201
Improving search order for reachability testing in timed automata
Standard algorithms for reachability analysis of timed automata are sensitive
to the order in which the transitions of the automata are taken. To tackle this
problem, we propose a ranking system and a waiting strategy. This paper
discusses the reason why the search order matters and shows how a ranking
system and a waiting strategy can be integrated into the standard reachability
algorithm to alleviate and prevent the problem respectively. Experiments show
that the combination of the two approaches gives optimal search order on
standard benchmarks except for one example. This suggests that it should be
used instead of the standard BFS algorithm for reachability analysis of timed
automata
On normalization-equivariance properties of supervised and unsupervised denoising methods: a survey
Image denoising is probably the oldest and still one of the most active
research topic in image processing. Many methodological concepts have been
introduced in the past decades and have improved performances significantly in
recent years, especially with the emergence of convolutional neural networks
and supervised deep learning. In this paper, we propose a survey of guided tour
of supervised and unsupervised learning methods for image denoising,
classifying the main principles elaborated during this evolution, with a
particular concern given to recent developments in supervised learning. It is
conceived as a tutorial organizing in a comprehensive framework current
approaches. We give insights on the rationales and limitations of the most
performant methods in the literature, and we highlight the common features
between many of them. Finally, we focus on on the normalization equivariance
properties that is surprisingly not guaranteed with most of supervised methods.
It is of paramount importance that intensity shifting or scaling applied to the
input image results in a corresponding change in the denoiser output
Fast algorithms for handling diagonal constraints in timed automata
A popular method for solving reachability in timed automata proceeds by
enumerating reachable sets of valuations represented as zones. A na\"ive
enumeration of zones does not terminate. Various termination mechanisms have
been studied over the years. Coming up with efficient termination mechanisms
has been remarkably more challenging when the automaton has diagonal
constraints in guards.
In this paper, we propose a new termination mechanism for timed automata with
diagonal constraints based on a new simulation relation between zones.
Experiments with an implementation of this simulation show significant gains
over existing methods.Comment: Shorter version of this article to appear in CAV 201
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
Normalization-Equivariant Neural Networks with Application to Image Denoising
In many information processing systems, it may be desirable to ensure that
any change of the input, whether by shifting or scaling, results in a
corresponding change in the system response. While deep neural networks are
gradually replacing all traditional automatic processing methods, they
surprisingly do not guarantee such normalization-equivariance (scale + shift)
property, which can be detrimental in many applications. To address this issue,
we propose a methodology for adapting existing neural networks so that
normalization-equivariance holds by design. Our main claim is that not only
ordinary convolutional layers, but also all activation functions, including the
ReLU (rectified linear unit), which are applied element-wise to the
pre-activated neurons, should be completely removed from neural networks and
replaced by better conditioned alternatives. To this end, we introduce
affine-constrained convolutions and channel-wise sort pooling layers as
surrogates and show that these two architectural modifications do preserve
normalization-equivariance without loss of performance. Experimental results in
image denoising show that normalization-equivariant neural networks, in
addition to their better conditioning, also provide much better generalization
across noise levels
Application of Partial-Order Methods to Reactive Systems with Event Memorization
International audienceWe are concerned in this paper with the verification of reactive systems with event memorization. The reactive systems are specified with an asynchronous reactive language Electre the main feature of which is the capability of memorizing occurrences of events in order to process them later. This memory capability is quite interesting for specifying reactive systems but leads to a verification model with a dramatically large number of states (due to the stored occurrences of events). In this paper, we show that partial-order methods can be applied successfuly for verification purposes on our model of reactive programs with event memorization. The main points of our work are two-fold: (1) we show that the independance relation which is a key point for applying partial-order methods can be extracted automatically from an \sf Electre program; (2) the partial-order technique turns out to be very efficient and may lead to a drastic reduction in the number of states of the model as demonstrated by a real-life industrial case study
Coarse abstractions make Zeno behaviours difficult to detect
An infinite run of a timed automaton is Zeno if it spans only a finite amount
of time. Such runs are considered unfeasible and hence it is important to
detect them, or dually, find runs that are non-Zeno. Over the years important
improvements have been obtained in checking reachability properties for timed
automata. We show that some of these very efficient optimizations make testing
for Zeno runs costly. In particular we show NP-completeness for the
LU-extrapolation of Behrmann et al. We analyze the source of this complexity in
detail and give general conditions on extrapolation operators that guarantee a
(low) polynomial complexity of Zenoness checking. We propose a slight weakening
of the LU-extrapolation that satisfies these conditions
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