146 research outputs found
Improving the mesomorphic behaviour of supramolecular liquid crystals by resonance-assisted hydrogen bonding
A systematic structure-property relationship study on hydrogen-bonded liquid crystals was performed, revealing the impact of resonance-assisted hydrogen bonds (RAHBs) on the self-assembling behavior of the supramolecular architecture. The creation of a six-membered intramolecular hydrogen-bonded ring acts as a counterpart to the self-organization between hydrogen bond donators and acceptors and determines thus the suprastructure. Variation of the hydrogen-bonding pattern allowed us to significantly improve the temperature range of the reported liquid crystalline assemblies
An Improvement of the Piggyback Algorithm for Parallel Model Checking
This paper extends the piggyback algorithm to enlarge the set of liveness properties it can verify. Its extension is motivated by an attempt to express in logic the counterexamples it can detect and relate them to bounded liveness. The original algorithm is based on parallel breadth-first search and piggybacking of accepting states that are deleted after counting a fixed number of transitions. The main improvement is obtained by renewing the counter of transitions when the same accepting states are visited in the negated property automaton. In addition, we describe piggybacking of multiple states in either sets (exact) or Bloom filters (lossy but conservative), and use of local searches that attempt to connect cycles fragmented among processing cores. Finally it is proved that accepting cycle detection is in NC in the size of the product automaton's entire state space, including unreachable states
Naturally occurring polyphenols as building blocks for supramolecular liquid crystals-substitution pattern dominates mesomorphism
A modular supramolecular approach towards hydrogen-bonded liquid crystalline assemblies based on naturally occurring polyphenols is reported. The combination of experimental observations, crystallographic studies and semi-empirical analyses of the assemblies provides insight into the structure-property relationships of these materials. Here a direct correlation of the number of donor OH-groups as well as their orientation with the mesomorphic behavior is reported. We discovered that the number and orientation of the OH-groups have a stronger influence on the mesomorphic behavior of the supramolecular assemblies than the connectivity (e.g. stilbenoid or chalconoid) of the hydrogen bond donors. Furthermore, the photo-switching behavior of selected complexes containing azopyridine ligands was investigated. This study will help future scientists to gain a deeper understanding of the underlying mechanisms and structure-property relationships of supramolecular assemblies with mesomorphic behavior, which is still one of the major challenges in current scienc
Omega-Regular Model Checking
peer reviewed"Regular model checking" is the name of a family of techniques for analyzing infinite-state systems in which states are represented by words or trees, sets of states by finite automata on these objects, and transitions by finite automata operating on pairs of state encodings, i.e. finite-state transducers. In this context, the central problem is then to compute the iterative closure of a finite-state transducer. This paper addresses the use of regular model-checking like techniques for systems whose states are represented by infinite (omega) words. Its main motivation is to show the feasibility and usefulness of this approach through a combination of the necessary theoretical developments, implementation, and experimentation. The iteration technique that is used is adapted from recent work of the authors on the iteration of finite-word transducers. It proceeds by comparing successive elements of a sequence of approximations of the iteration, detecting an "increment" that is added to move from one approximation to the next, and extrapolating the sequence by allowing arbitrary repetitions of this increment. By restricting oneself to weak deterministic Buchi automata, and using a number of implementation optimizations, examples of significant size can be handled. The proposed transducer iteration technique can just as well be exploited to compute the closure of a given set of states by the transducer iteration, which has proven to be a very effective way of using the technique. Examples such as a leaking gas burner in which time is modeled by real variables have been handled completely within the automata-theoretic setting
Two Variable vs. Linear Temporal Logic in Model Checking and Games
Model checking linear-time properties expressed in first-order logic has
non-elementary complexity, and thus various restricted logical languages are
employed. In this paper we consider two such restricted specification logics,
linear temporal logic (LTL) and two-variable first-order logic (FO2). LTL is
more expressive but FO2 can be more succinct, and hence it is not clear which
should be easier to verify. We take a comprehensive look at the issue, giving a
comparison of verification problems for FO2, LTL, and various sublogics thereof
across a wide range of models. In particular, we look at unary temporal logic
(UTL), a subset of LTL that is expressively equivalent to FO2; we also consider
the stutter-free fragment of FO2, obtained by omitting the successor relation,
and the expressively equivalent fragment of UTL, obtained by omitting the next
and previous connectives. We give three logic-to-automata translations which
can be used to give upper bounds for FO2 and UTL and various sublogics. We
apply these to get new bounds for both non-deterministic systems (hierarchical
and recursive state machines, games) and for probabilistic systems (Markov
chains, recursive Markov chains, and Markov decision processes). We couple
these with matching lower-bound arguments. Next, we look at combining FO2
verification techniques with those for LTL. We present here a language that
subsumes both FO2 and LTL, and inherits the model checking properties of both
languages. Our results give both a unified approach to understanding the
behaviour of FO2 and LTL, along with a nearly comprehensive picture of the
complexity of verification for these logics and their sublogics.Comment: 37 pages, to be published in Logical Methods in Computer Science
journal, includes material presented in Concur 2011 and QEST 2012 extended
abstract
Tableau-based decision procedures for logics of strategic ability in multi-agent systems
We develop an incremental tableau-based decision procedures for the
Alternating-time temporal logic ATL and some of its variants.
While running within the theoretically established complexity upper bound, we
claim that our tableau is practically more efficient in the average case than
other decision procedures for ATL known so far. Besides, the ease of its
adaptation to variants of ATL demonstrates the flexibility of the proposed
procedure.Comment: To appear in ACM Transactions on Computational Logic. 48 page
Formal methods and tools for the development of distributed and real time systems : Esprit Project 3096 (SPEC)
The Basic Research Action No. 3096, Formal Methods snd Tools for the Development of Distributed and Real Time Systems, is funded in the Area of Computer Science, under the ESPRIT Programme of the European Community. The coordinating institution is the Department of Computing Science, Eindhoven University of Technology, and the participating Institutions are the Institute of Computer Science of Crete. the Swedish Institute of Computer Science, the Programmimg Research Group of the University of Oxford, and the Computer Science Departments of the University of Manchester, Imperial
College. Weizmann Institute of Science, Eindhoven University of Technology, IMAG Grenoble. Catholic University of Nijmegen, and the University of Liege. This document contains the synopsis. and part of the sections on objectives and area of advance, on baseline and rationale, on research goals, and on organisation of the action, as contained in the original proposal, submitted June, 198S. The section on the state of the art (18 pages) and the full list of references (21 pages) of the original proposal have been deleted because of limitation of available space
A theory of normed simulations
In existing simulation proof techniques, a single step in a lower-level
specification may be simulated by an extended execution fragment in a
higher-level one. As a result, it is cumbersome to mechanize these techniques
using general purpose theorem provers. Moreover, it is undecidable whether a
given relation is a simulation, even if tautology checking is decidable for the
underlying specification logic. This paper introduces various types of normed
simulations. In a normed simulation, each step in a lower-level specification
can be simulated by at most one step in the higher-level one, for any related
pair of states. In earlier work we demonstrated that normed simulations are
quite useful as a vehicle for the formalization of refinement proofs via
theorem provers. Here we show that normed simulations also have pleasant
theoretical properties: (1) under some reasonable assumptions, it is decidable
whether a given relation is a normed forward simulation, provided tautology
checking is decidable for the underlying logic; (2) at the semantic level,
normed forward and backward simulations together form a complete proof method
for establishing behavior inclusion, provided that the higher-level
specification has finite invisible nondeterminism.Comment: 31 pages, 10figure
Model Checking CTL is Almost Always Inherently Sequential
The model checking problem for CTL is known to be P-complete (Clarke,
Emerson, and Sistla (1986), see Schnoebelen (2002)). We consider fragments of
CTL obtained by restricting the use of temporal modalities or the use of
negations---restrictions already studied for LTL by Sistla and Clarke (1985)
and Markey (2004). For all these fragments, except for the trivial case without
any temporal operator, we systematically prove model checking to be either
inherently sequential (P-complete) or very efficiently parallelizable
(LOGCFL-complete). For most fragments, however, model checking for CTL is
already P-complete. Hence our results indicate that, in cases where the
combined complexity is of relevance, approaching CTL model checking by
parallelism cannot be expected to result in any significant speedup. We also
completely determine the complexity of the model checking problem for all
fragments of the extensions ECTL, CTL+, and ECTL+
Extended Computation Tree Logic
We introduce a generic extension of the popular branching-time logic CTL
which refines the temporal until and release operators with formal languages.
For instance, a language may determine the moments along a path that an until
property may be fulfilled. We consider several classes of languages leading to
logics with different expressive power and complexity, whose importance is
motivated by their use in model checking, synthesis, abstract interpretation,
etc.
We show that even with context-free languages on the until operator the logic
still allows for polynomial time model-checking despite the significant
increase in expressive power. This makes the logic a promising candidate for
applications in verification.
In addition, we analyse the complexity of satisfiability and compare the
expressive power of these logics to CTL* and extensions of PDL
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