Results 1  10
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127
The Theory of Hybrid Automata
, 1996
"... A hybrid automaton is a formal model for a mixed discretecontinuous system. We classify hybrid automata acoording to what questions about their behavior can be answered algorithmically. The classification reveals structure on mixed discretecontinuous state spaces that was previously studied on pur ..."
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Cited by 483 (9 self)
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A hybrid automaton is a formal model for a mixed discretecontinuous system. We classify hybrid automata acoording to what questions about their behavior can be answered algorithmically. The classification reveals structure on mixed discretecontinuous state spaces that was previously studied on purely discrete state spaces only. In particular, various classes of hybrid automata induce finitary trace equivalence (or similarity, or bisimilarity) relations on an uncountable state space, thus permitting the application of various modelchecking techniques that were originally developed for finitestate systems.
Index Structures for Path Expressions
, 1997
"... In recent years there has been an increased interest in managing data which does not conform to traditional data models, like the relational or object oriented model. The reasons for this nonconformance are diverse. One one hand, data may not conform to such models at the physical level: it may be ..."
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Cited by 283 (8 self)
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In recent years there has been an increased interest in managing data which does not conform to traditional data models, like the relational or object oriented model. The reasons for this nonconformance are diverse. One one hand, data may not conform to such models at the physical level: it may be stored in data exchange formats, fetched from the Internet, or stored as structured les. One the other hand, it may not conform at the logical level: data may have missing attributes, some attributes may be of di erent types in di erent data items, there may be heterogeneous collections, or the data may be simply specified by a schema which is too complex or changes too often to be described easily as a traditional schema. The term semistructured data has been used to refer to such data. The data model proposed for this kind of data consists of an edgelabeled graph, in which nodes correspond to objects and edges to attributes or values. Figure 1 illustrates a semistructured database providing information about a city. Relational databases are traditionally queried with associative queries, retrieving tuples based on the value of some attributes. To answer such queries efciently, database management systems support indexes for translating attribute values into tuple ids (e.g. Btrees or hash tables). In objectoriented databases, path queries replace the simpler associative queries. Several data structures have been proposed for answering path queries e ciently: e.g., access support relations 14] and path indexes 4]. In the case of semistructured data, queries are even more complex, because they may contain generalized path expressions 1, 7, 8, 16]. The additional exibility is needed in order to traverse data whose structure is irregular, or partially unknown to the user.
Adding structure to unstructured data
 In 6th Int. Conf. on Database Theory (ICDT ’97),LNCS 1186, 336–350
, 1997
"... We develop a new schema for unstructured data. Traditional schemas resemble the type systems of programming languages. For unstructured data, however, the underlying type may be much less constrained and hence an alternative way of expressing constraints on the data is needed. Here, we propose that ..."
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Cited by 204 (22 self)
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We develop a new schema for unstructured data. Traditional schemas resemble the type systems of programming languages. For unstructured data, however, the underlying type may be much less constrained and hence an alternative way of expressing constraints on the data is needed. Here, we propose that both data and schema be represented as edgelabeled graphs. We develop notions of conformance between a graph database and a graph schema and show that there is a natural and e ciently computable ordering on graph schemas. We then examine certain subclasses of schemas and show that schemas are closed under query applications. Finally, we discuss how they may be used in query decomposition and optimization. 1
Alternating refinement relations
 In Proceedings of the Ninth International Conference on Concurrency Theory (CONCUR’98), volume 1466 of LNCS
, 1998
"... Abstract. Alternating transition systems are a general model for composite systems which allow the study of collaborative as well as adversarial relationships between individual system components. Unlike in labeled transition systems, where each transition corresponds to a possible step of the syste ..."
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Cited by 116 (16 self)
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Abstract. Alternating transition systems are a general model for composite systems which allow the study of collaborative as well as adversarial relationships between individual system components. Unlike in labeled transition systems, where each transition corresponds to a possible step of the system (which may involve some or all components), in alternating transition systems, each transition corresponds to a possible move in a game between the components. In this paper, we study refinement relations between alternating transition systems, such as “Does the implementation refine the set £ of specification components without constraining the components not in £? ” In particular, we generalize the definitions of the simulation and trace containment preorders from labeled transition systems to alternating transition systems. The generalizations are called alternating simulation and alternating trace containment. Unlike existing refinement relations, they allow the refinement of individual components within the context of a composite system description. We show that, like ordinary simulation, alternating simulation can be checked in polynomial time using a fixpoint computation algorithm. While ordinary trace containment is PSPACEcomplete, we establish alternating trace containment to be EXPTIMEcomplete. Finally, we present logical characterizations for the two preorders in terms of ATL, a temporal logic capable of referring to games between system components. 1
UnQL: A Query Language and Algebra for Semistructured Data Based on Structural Recursion
, 2000
"... This paper presents structural recursion as the basis of the syntax and semantics of query languages for semistructured data and XML. We describe a simple and powerful query language based on pattern matching and show that it can be expressed using structural recursion, which is introduced as a top ..."
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Cited by 107 (4 self)
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This paper presents structural recursion as the basis of the syntax and semantics of query languages for semistructured data and XML. We describe a simple and powerful query language based on pattern matching and show that it can be expressed using structural recursion, which is introduced as a topdown, recursive function, similar to the way XSL is defined on XML trees. On cyclic data, structural recursion can be defined in two equivalent ways: as a recursive function which evaluates the data topdown and remembers all its calls to avoid infinite loops, or as a bulk evaluation which processes the entire data in parallel using only traditional relational algebra operators. The latter makes it possible for optimization techniques in relational queries to be applied to structural recursion. We show that the composition of two structural recursion queries can be expressed as a single such query, and this is used as the basis of an optimization method for mediator systems. Several other fo...
Computing Least Common Subsumers in Description Logics with Existential Restrictions
, 1999
"... Computing the least common subsumer (lcs) is an inference task that can be used to support the "bottomup " construction of knowledge bases for KR systems based on description logics. Previous work on how to compute the lcs has concentrated on description logics that allow for universal va ..."
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Cited by 89 (24 self)
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Computing the least common subsumer (lcs) is an inference task that can be used to support the "bottomup " construction of knowledge bases for KR systems based on description logics. Previous work on how to compute the lcs has concentrated on description logics that allow for universal value restrictions, but not for existential restrictions. The main new contribution of this paper is the treatment of description logics with existential restrictions. Our approach for computing the lcs is based on an appropriate representation of concept descriptions by certain trees, and a characterization of subsumption by homomorphisms between these trees. The lcs operation then corresponds to the product operation on trees.
Least Common Subsumers and Most Specific Concepts in a Description Logic with Existential Restrictions and Terminological Cycles
, 2003
"... Computing least common subsumers (Ics) and most specific concepts (msc) are inference tasks that can support the bottomup construction of knowledge bases in description logics. In description logics with existential restrictions, the most specific concept need not exist if one restricts the attenti ..."
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Cited by 72 (17 self)
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Computing least common subsumers (Ics) and most specific concepts (msc) are inference tasks that can support the bottomup construction of knowledge bases in description logics. In description logics with existential restrictions, the most specific concept need not exist if one restricts the attention to concept descriptions or acyclic TBoxes. In this paper, we extend the notions les and msc to cyclic TBoxes. For the description logic EC (which allows for conjunctions, existential restrictions, and the topconcept), we show that the les and msc always exist and can be computed in polynomial time if we interpret cyclic definitions with greatest fixpoint semantics.
Race Checking by Context Inference
 In PLDI
, 2004
"... Software model checking has been successful for sequential programs, where predicate abstraction offers suitable models, and counterexampleguided abstraction refinement permits the automatic inference of models. When checking concurrent programs, we need to abstract threads as well as the contexts ..."
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Cited by 71 (3 self)
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Software model checking has been successful for sequential programs, where predicate abstraction offers suitable models, and counterexampleguided abstraction refinement permits the automatic inference of models. When checking concurrent programs, we need to abstract threads as well as the contexts in which they execute. Stateless context models, such as predicates on global variables, prove insufficient for showing the absence of race conditions in many examples. We therefore use richer context models, which combine (1) predicates for abstracting data state, (2) control ow quotients for abstracting control state, and (3) counters for abstracting an unbounded number of threads. We infer suitable context models automatically by a combination of counterexampleguided abstraction refinement, bisimulation minimization, circular assumeguarantee reasoning, and parametric reasoning about an unbounded number of threads. This algorithm, called CIRC, has been implemented in Blast and succeeds in checking many examples of nesC code for data races. In particular, Blast proves the absence of races in several cases where previous race checkers give false positives.
Optimizing Büchi automata
, 2000
"... We describe a family of optimizations implemented in a translation from a linear temporal logic to Büchi automata. Such optimized automata can enhance the efficiency of explicit state model checking, as practiced in tools such as SPIN. Some of our optimizations are applicable to... ..."
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Cited by 57 (3 self)
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We describe a family of optimizations implemented in a translation from a linear temporal logic to Büchi automata. Such optimized automata can enhance the efficiency of explicit state model checking, as practiced in tools such as SPIN. Some of our optimizations are applicable to...