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48
Verification using simulation
 In: Hybrid Systems: Computation and Control (HSCC). Volume 3927 of LNCS., Springer (2006) 272 – 286
, 2006
"... Abstract. Verification and simulation have always been complementary, if not competing, approaches to system design. In this paper, we present a novel method for socalled metric transition systems that bridges the gap between verification and simulation, enabling system verification using a finite ..."
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Cited by 44 (6 self)
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Abstract. Verification and simulation have always been complementary, if not competing, approaches to system design. In this paper, we present a novel method for socalled metric transition systems that bridges the gap between verification and simulation, enabling system verification using a finite number of simulations. The existence of metrics on the system state and observation spaces, which is natural for continuous systems, allows us to capitalize on the recently developed framework of approximate bisimulations, and infer the behavior of neighborhood of system trajectories around a simulated trajectory. For nondeterministic linear systems that are robustly safe or robustly unsafe, we provide not only a completeness result but also an upper bound on the number of simulations required as a function of the distance between the reachable set and the unsafe set. Our framework is the first simulationbased verification method that enjoys completeness for infinitestate systems. The complexity is low for robustly safe or robustly unsafe systems, and increases for nonrobust problems. This provides strong evidence that robustness dramatically impacts the complexity of system verification and design. 1
Exposición General
 Revista Internacional del Trabajo
, 1995
"... Twitter offers an explicit mechanism to facilitate information diffusion and has emerged as a new medium for communication. Many approaches to find influentials have been proposed, but they do not consider the temporal order of information adoption. In this work, we propose a novel method to find in ..."
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Cited by 25 (1 self)
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Twitter offers an explicit mechanism to facilitate information diffusion and has emerged as a new medium for communication. Many approaches to find influentials have been proposed, but they do not consider the temporal order of information adoption. In this work, we propose a novel method to find influentials by considering both the link structure and the temporal order of information adoption in Twitter. Our method finds distinct influentials who are not discovered by other methods.
Modeling Timed Concurrent Systems
 in CONCUR 2006  Concurrency Theory
, 2006
"... Abstract. Timed concurrent systems are widely used in concurrent and distributed realtime software, modeling of hybrid systems, design of hardware systems (using hardware description languages), discreteevent simulation, and modeling of communication networks. They consist of concurrent components ..."
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Cited by 21 (18 self)
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Abstract. Timed concurrent systems are widely used in concurrent and distributed realtime software, modeling of hybrid systems, design of hardware systems (using hardware description languages), discreteevent simulation, and modeling of communication networks. They consist of concurrent components that communicate using timed signals, that is, sets of (semantically) timestamped events. The denotational semantics of such systems is traditionally formulated in a metric space, wherein causal components are modeled as contracting functions. We show that this formulation excessively restricts the models of time that can be used. In particular, it cannot handle superdense time, commonly used in hardware description languages and hybrid systems modeling, finite time lines, and time with no origin. Moreover, if we admit continuoustime and mixed signals (essential for hybrid systems modeling) or certain Zeno signals, then causality is no longer equivalent to its formalization in terms of contracting functions. In this paper, we offer an alternative semantic framework using a generalized ultrametric that overcomes these limitations. 1
Causality Interfaces and Compositional Causality Analysis
 FIT 2005 PRELIMINARY VERSION
, 2005
"... In this paper, we consider concurrent models of computation where ”actors” (components that are in charge of their own actions) communicate by exchanging messages. The interfaces of actors principally consist of “ports,” which mediate the exchange of messages. Actororiented architectures contrast w ..."
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Cited by 15 (10 self)
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In this paper, we consider concurrent models of computation where ”actors” (components that are in charge of their own actions) communicate by exchanging messages. The interfaces of actors principally consist of “ports,” which mediate the exchange of messages. Actororiented architectures contrast with and complement objectoriented models by emphasizing the exchange of data between concurrent components rather than transfer of control. Examples of such models of computation include the classical actor model, synchronous languages, dataflow models, and discreteevent models. Many of these models of computation benefit considerably from having access to causality information about the components. This paper augments the interfaces of such components to include such causality information. It shows how this causality information can be algebraically composed so that compositions of components acquire causality interfaces that are inferred from their components and the interconnections. We illustrate the use of these causality interfaces to statically analyze discreteevent models for uniqueness of behaviors, synchronous models for causality loops, and dataflow models for schedulability.
A Constructive FixedPoint Theorem and the Feedback Semantics of Timed Systems
 in Workshop on Discrete Event Systems (WODES), Ann Arbor
, 2006
"... Abstract — Deterministic timed systems can be modeled as fixed point problems [15], [16], [4]. In particular, any connected network of timed systems can be modeled as a single system with feedback, and the system behavior is the fixed point of the corresponding system equation, when it exists. For d ..."
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Cited by 12 (11 self)
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Abstract — Deterministic timed systems can be modeled as fixed point problems [15], [16], [4]. In particular, any connected network of timed systems can be modeled as a single system with feedback, and the system behavior is the fixed point of the corresponding system equation, when it exists. For deltacausal systems, we can use the Cantor metric to measure the distance between signals and the Banach fixedpoint theorem to prove the existence and uniqueness of a system behavior. Moreover, the Banach fixedpoint theorem is constructive: it provides a method to construct the unique fixed point through iteration. In this paper, we extend this result to systems modeled with the superdense model of time [7], [8] used in hybrid systems. We call the systems we consider eventually deltacausal, a strict generalization of deltacausal in which multiple events may be generated on a signal in zero time. With this model of time, we can use a generalized ultrametric [14] instead of a metric to model the distance between signals. The existence and uniqueness of behaviors for such systems comes from the fixedpoint theorem of [13], but this theorem gives no constructive method to compute the fixed point. This leads us to define petrics, a generalization of metrics, which we use to generalize the Banach fixedpoint theorem to provide a constructive fixedpoint theorem. This new fixedpoint theorem allows us to construct the unique behavior of eventually deltacausal systems. I.
Modal Models in Ptolemy
, 2010
"... Ptolemy is an opensource and extensible modeling and simulation framework. It offers heterogeneous modeling capabilities by allowing different models of computation to be composed hierarchically in an arbitrary fashion. This paper describes modal models, which allow to hierarchically compose finite ..."
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Cited by 10 (8 self)
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Ptolemy is an opensource and extensible modeling and simulation framework. It offers heterogeneous modeling capabilities by allowing different models of computation to be composed hierarchically in an arbitrary fashion. This paper describes modal models, which allow to hierarchically compose finitestate machines with other models of computation, both untimed and timed. The semantics of modal models in Ptolemy are defined in a modular manner.
Sensitivity, approximation and uncertainty in power system dynamic simulation
 IEEE TRANSACTIONS ON POWER SYSTEMS, SUBMITTED
, 2006
"... Parameters of power system models, in particular load models, are seldom known exactly. Yet dynamic security assessment relies upon simulation of those uncertain models. This paper proposes a computationally feasible approach to assessing the influence of uncertainty in simulations of power system d ..."
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Cited by 10 (4 self)
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Parameters of power system models, in particular load models, are seldom known exactly. Yet dynamic security assessment relies upon simulation of those uncertain models. This paper proposes a computationally feasible approach to assessing the influence of uncertainty in simulations of power system dynamic behaviour. It is shown that trajectory sensitivities can be used to generate accurate firstorder approximations of trajectories that arise from perturbed parameter sets. The computational cost of obtaining the sensitivities and perturbed trajectories is minimal. The mathematical structure of the trajectory approximations allows the effects of uncertainty to be quantified and visualized using worstcase analysis and probabilistic approaches.
Divide and recycle: types and compilation for a hybrid synchronous language
 In ACM SIGPLAN/SIGBED Languages, Compilers, Tools & Theory for Embedded Systems (LCTES
, 2011
"... Hybrid modelers such as SIMULINK have become corner stones of embedded systems development. They allow both discrete controllers and their continuous environments to be expressed in a single language. Despite the availability of such tools, there remain a number of issues related to the lack of repr ..."
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Cited by 7 (2 self)
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Hybrid modelers such as SIMULINK have become corner stones of embedded systems development. They allow both discrete controllers and their continuous environments to be expressed in a single language. Despite the availability of such tools, there remain a number of issues related to the lack of reproducibility of simulations and to the separation of the continuous part, which has to be exercised by a numerical solver, from the discrete part, which must be guaranteed not to evolve during a step. Starting from a minimal, yet fullfeatured, LUSTRElike synchronous language, this paper presents a conservative extension where dataflow equations can be mixed with ordinary differential equations (ODEs) with possible reset. A type system is proposed to statically distinguish discrete computations from continuous ones and to ensure that signals are used in their proper domains. We