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17
Computing differential invariants of hybrid systems as fixedpoints
, 2008
"... Abstract. We introduce a fixedpoint algorithm for verifying safety properties of hybrid systems with differential equations whose righthand sides are polynomials in the state variables. In order to verify nontrivial systems without solving their differential equations and without numerical errors, ..."
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Cited by 58 (21 self)
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Abstract. We introduce a fixedpoint algorithm for verifying safety properties of hybrid systems with differential equations whose righthand sides are polynomials in the state variables. In order to verify nontrivial systems without solving their differential equations and without numerical errors, we use a continuous generalization of induction, for which our algorithm computes the required differential invariants. As a means for combining local differential invariants into global system invariants in a sound way, our fixedpoint algorithm works with a compositional verification logic for hybrid systems. To improve the verification power, we further introduce a saturation procedure that refines the system dynamics successively with differential invariants until safety becomes provable. By complementing our symbolic verification algorithm with a robust version of numerical falsification, we obtain a fast and sound verification procedure. We verify roundabout maneuvers in air traffic management and collision avoidance in train control.
DifferentialAlgebraic Dynamic Logic for DifferentialAlgebraic Programs
"... Abstract. We generalise dynamic logic to a logic for differentialalgebraic programs, i.e., discrete programs augmented with firstorder differentialalgebraic formulas as continuous evolution constraints in addition to firstorder discrete jump formulas. These programs characterise interacting discr ..."
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Cited by 41 (28 self)
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Abstract. We generalise dynamic logic to a logic for differentialalgebraic programs, i.e., discrete programs augmented with firstorder differentialalgebraic formulas as continuous evolution constraints in addition to firstorder discrete jump formulas. These programs characterise interacting discrete and continuous dynamics of hybrid systems elegantly and uniformly. For our logic, we introduce a calculus over real arithmetic with discrete induction and a new differential induction with which differentialalgebraic programs can be verified by exploiting their differential constraints algebraically without having to solve them. We develop the theory of differential induction and differential refinement and analyse their deductive power. As a case study, we present parametric tangential roundabout maneuvers in air traffic control and prove collision avoidance in our calculus.
ConstraintBased Approach for Analysis of Hybrid Systems
 of Lecture Notes in Computer Science
, 2008
"... Abstract. This paper presents a constraintbased technique for discovering a rich class of inductive invariants (disjunctions of polynomial inequalities of bounded degree) for verification of hybrid systems. The key idea is to introduce a template for the unknown invariants and then translate the ve ..."
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Cited by 41 (10 self)
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Abstract. This paper presents a constraintbased technique for discovering a rich class of inductive invariants (disjunctions of polynomial inequalities of bounded degree) for verification of hybrid systems. The key idea is to introduce a template for the unknown invariants and then translate the verification condition of the hybrid system into an ∃ ∀ constraint over the template unknowns (which are variables over reals) by making use of the fact that vector fields must point inwards at the boundary. These constraints are then solved using Farkas lemma. We also present preliminary experimental results that demonstrate the feasibility of our approach of solving the ∃ ∀ constraints generated from models of realworld hybrid systems. 1
Symbolic model checking of hybrid systems using template polyhedra
 In TACAS’08  Tools and Algorithms for
, 2008
"... Abstract. We propose techniques for the verification of hybrid systems using template polyhedra, i.e., polyhedra whose inequalities have fixed expressions but with varying constant terms. Given a hybrid system description and a set of template linear expressions as inputs, our technique constructs o ..."
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Cited by 20 (7 self)
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Abstract. We propose techniques for the verification of hybrid systems using template polyhedra, i.e., polyhedra whose inequalities have fixed expressions but with varying constant terms. Given a hybrid system description and a set of template linear expressions as inputs, our technique constructs overapproximations of the reachable states using template polyhedra. Therefore, operations used in symbolic model checking such as intersection, union and postcondition across discrete transitions over template polyhedra can be computed efficiently using template polyhedra without requiring expensive vertex enumeration. Additionally, the verification of hybrid systems requires techniques to handle the continuous dynamics inside discrete modes. We propose a new flowpipe construction algorithm using template polyhedra. Our technique uses higherorder Taylor series expansion to approximate the time trajectories. The terms occurring in the Taylor series expansion are bounded using repeated optimization queries. The location invariant is used to enclose the remainder term of the Taylor series, and thus truncate the expansion. Finally, we have implemented our technique as a part of the tool TimePass for the analysis of affine hybrid automata. 1
Event count automata: A statebased model for stream processing systems
 IN RTSS
, 2005
"... Recently there has been a growing interest in models and methods targeted towards the (co)design of stream processing applications; e.g. those for audio/video processing. Streams processed by such applications tend to be highly bursty and exhibit a high datadependent variability in their processing ..."
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Cited by 18 (7 self)
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Recently there has been a growing interest in models and methods targeted towards the (co)design of stream processing applications; e.g. those for audio/video processing. Streams processed by such applications tend to be highly bursty and exhibit a high datadependent variability in their processing requirements. As a result, classical event and service models such as periodic, sporadic, etc. can be overly pessimistic when dealing with such applications. In this paper, we present a new model called Event Count Automata (ECA) for capturing the timing properties of such streams. Our model can be used to cleanly formulate properties relevant to stream processing on heterogeneous multiprocessor architectures, such as buffer overflow/underflow constraints. It can also provide the basis for developing analysis methods to compute delay/timing properties of the processed streams under different scheduling policies. Our ECAs, though similar in flavor to timed and hybrid automata, have a different semantics, are more lightweight, and are specifically suited for modeling stream processing applications and architectures. We present the basic aspects of this model and illustrate its modeling potential. We then apply it in a specific stream processing setting and develop an analysis technique based on the formalism of Colored Petri Nets (CPNs). Finally, we validate our modeling and analysis techniques with the help of preliminary experimental results generated using the CPN simulation tool.
Providing a basin of attraction to a target region by computation of Lyapunovlike functions
 In IEEE Int. Conf. on Computational Cybernetics
, 2006
"... Abstract — In this paper, we present a method for computing a basin of attraction to a target region for nonlinear ordinary differential equations. This basin of attraction is ensured by a Lyapunovlike polynomial function that we compute using an interval based branchandrelax algorithm. This alg ..."
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Cited by 11 (5 self)
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Abstract — In this paper, we present a method for computing a basin of attraction to a target region for nonlinear ordinary differential equations. This basin of attraction is ensured by a Lyapunovlike polynomial function that we compute using an interval based branchandrelax algorithm. This algorithm relaxes the necessary conditions on the coefficients of the Lyapunovlike function to a system of linear interval inequalities that can then be solved exactly, and iteratively reduces the relaxation error by recursively decomposing the state space into hyperrectangles. Tests on an implementation are promising. I.
Finding Polynomial Invariants for Imperative Loops in the Theorema System
, 2006
"... Abstract. We present an algorithm for finding valid polynomial relations (i. e. invariants) among program variables for imperative loops. The algorithm is implemented in the verification environment for imperative programs (using Hoare logic) in the frame of the Theorema system (www.theorema.org). W ..."
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Cited by 8 (4 self)
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Abstract. We present an algorithm for finding valid polynomial relations (i. e. invariants) among program variables for imperative loops. The algorithm is implemented in the verification environment for imperative programs (using Hoare logic) in the frame of the Theorema system (www.theorema.org). We use techniques from (polynomial) algebra and combinatorics, namely Gröbner Bases, variable elimination, algebraic dependencies and symbolic summation (the Gosper algorithm, handling geometric series, Cfinite solving). These methods are demonstrated on several examples which have been treated completely automatically by our implementation.
M.: Exponentialconditionbased barrier certificate generation for safety verification of hybrid systems
 In: CAV’13
, 2013
"... Abstract. A barrier certificate is an inductive invariant function which can be used for the safety verification of a hybrid system. Safety verification based on barrier certificate has the benefit of avoiding explicit computation of the exact reachable set which is usually intractable for nonlinea ..."
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Cited by 4 (0 self)
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Abstract. A barrier certificate is an inductive invariant function which can be used for the safety verification of a hybrid system. Safety verification based on barrier certificate has the benefit of avoiding explicit computation of the exact reachable set which is usually intractable for nonlinear hybrid systems. In this paper, we propose a new barrier certificate condition, called Exponential Condition, for the safety verification of semialgebraic hybrid systems. The most important benefit of Exponential Condition is that it has a lower conservativeness than the existing convex condition and meanwhile it possesses the property of convexity. On the one hand, a less conservative barrier certificate forms a tighter overapproximation for the reachable set and hence is able to verify critical safety properties. On the other hand, the property of convexity guarantees its solvability by semidefinite programming method. Some examples are presented to illustrate the effectiveness and practicality of our method.
Characterizing Algebraic Invariants by Differential Radical Invariants ⋆
"... Abstract We prove that any invariant algebraic set of a given polynomial vector field can be algebraically represented by one polynomial and a finite set of its successive Lie derivatives. This socalled differential radical characterization relies on a sound abstraction of the reachable set of solu ..."
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Cited by 4 (2 self)
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Abstract We prove that any invariant algebraic set of a given polynomial vector field can be algebraically represented by one polynomial and a finite set of its successive Lie derivatives. This socalled differential radical characterization relies on a sound abstraction of the reachable set of solutions by the smallest variety that contains it. The characterization leads to a differential radical invariant proof rule that is sound and complete, which implies that invariance of algebraic equations over realclosed fields is decidable. Furthermore, the problem of generating invariant varieties is shown to be as hard as minimizing the rank of a symbolic matrix, and is therefore NPhard. We investigate symbolic linear algebra tools based on Gaussian elimination to efficiently automate the generation. The approach can, e.g., generate nontrivial algebraic invariant equations capturing the airplane behavior during takeoff or landing in longitudinal motion.
Generating Box Invariants
"... Box invariant sets are boxshaped positively invariant sets. We show that box invariants are computable for a large class of nonlinear and hybrid systems. The technique for computing these invariants is based on nonlinear constraint solving. This paper also shows that the class of multiaffine syst ..."
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Cited by 3 (2 self)
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Box invariant sets are boxshaped positively invariant sets. We show that box invariants are computable for a large class of nonlinear and hybrid systems. The technique for computing these invariants is based on nonlinear constraint solving. This paper also shows that the class of multiaffine systems, which has been used successfully for modeling and analyzing regulatory and biochemical reaction networks, can be generalized to the class of componentwise monotone and componentwise quasi monotone systems without losing any of its nice properties.