14 research outputs found

    Reasoning Algebraically with Description Logics

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    Semantic Web applications based on the Web Ontology Language (OWL) often require the use of numbers in class descriptions for expressing cardinality restrictions on properties or even classes. Some of these cardinalities are specified explicitly, but quite a few are entailed and need to be discovered by reasoning procedures. Due to the Description Logic (DL) foundation of OWL, those reasoning services are offered by DL reasoners. Existing DL reasoners employ reasoning procedures that are arithmetically uninformed and substitute arithmetic reasoning by "don't know" non-determinism in order to cover all possible cases. This lack of information about arithmetic problems dramatically degrades the performance of DL reasoners in many cases, especially with ontologies relying on the use of Nominals and Qualied Cardinality Restrictions. The contribution of this thesis is twofold: on the theoretical level, it presents algebraļæ½ic reasoning with DL (ReAl DL) using a sound, complete, and terminating reasoning procedure for the DL SHOQ. ReAl DL combines tableau reasoning procedures with algebraic methods, namely Integer Programming, to ensure arithmetically better informed reasoning. SHOQ extends the standard DL ALC with transitive roles, role hierarchies, qualified cardinality restrictions (QCRs), and nominals, and forms an expressive subset of OWL. Although the proposed algebraic tableau is double exponential in the worst case, it deals with cardinalities with an additional level of information and properties that make the calculus amenable and well suited for optimizations. In order for ReAl DL to have a practical merit, suited optimizations are proposed towards achieving an efficient reasoning approach that addresses the sources of complexity related to nominals and QCRs. On the practical level, a running prototype reasoner (HARD) is implemented based on the proposed calculus and optimizations. HARD is used to evaluate the practical merit of ReAl DL, as well as the effectiveness of the proposed optimizations. Experimental results based on real world and synthetic ontologies show that ReAl DL outperforms existing reasoning approaches in handling the interactions between nominals and QCRs. ReAl DL also comes with some interesting features such as the ability to handle ontologies with cyclic descriptions without adopting special blocking strategies. ReAl DL can form a basis to provide more efficient reasoning support for ontologies using nominals or QCRs

    Ordering, Indexing, and Searching Semantic Data: A Terminology Aware Index Structure

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    Indexing data for efficient search capabilities is a core problem in many domains of computer science. As applications centered around semantic data sources become more common, the need for more sophisticated indexing and querying capabilities arises. In particular, the need to search for specific information in the presence of a terminology or ontology (i.e. a set of logic based rules that describe concepts and their relations) becomes of particular importance, as the information a user seeks may exists as an entailment of the explicit data by means of the terminology. This variant on traditional indexing and search problems forms the foundation of a range of possible technologies for semantic data. In this work, we propose an ordering language for specifying partial orders over semantic data items modeled as descriptions in a description logic. We then show how these orderings can be used as the basis of a search tree index for processing \emph{concept searches} in the presence of a terminology. We study in detail the properties of the orderings and the associated index structure, and also explore a relationship between ordering descriptions called \emph{order refinement}. A sound and complete procedure for deciding refinement is given. We also empirically evaluate a prototype implementation of our index structure, validating its potential efficacy in semantic query problems

    Modularity Through Inseparability : Algorithms, Extensions, and Evaluation

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    Module extraction is the task of computing, given a description logic ontology and a signature āˆ‘ of interest, a subset (called a module) such that for certain applications that only concern āˆ‘ the ontology can be equivalently replaced by the module. In most applications of module extraction it is desirable to compute a module which is as small as possible, and where possible a minimal one. In logic-based approaches to module extraction the most popular way to define modules is using inseparability relations, the strongest and most robust notion of this being model āˆ‘-inseparability, where two ontologies are called āˆ‘-inseparable iff the āˆ‘-reducts of their models coincide. Then, a āˆ‘-module is defined as a āˆ‘-inseparable subset of the ontology. Unfortunately deciding if a subset of an ontology is a minimal āˆ‘-module, over ontologies formulated in even moderately expressive logics, is of perpetually high complexity and often undecidable, and for this reason approximation algorithms are required. Instead of computing a minimal āˆ‘-module one computes some āˆ‘-module and the main research task is to minimise the size of these modules --- to compute an approximation of a minimal āˆ‘-module. This thesis considers research surrounding approximations based on the model āˆ‘-inseparability relation including: improving and extending existing approximation algorithms, providing a highly-optimised implementations, and the introduction a new methodology to evaluate just how well approximations approximate minimal modules, all supported by a significant empirical investigation

    Proceedings of the 26th International Symposium on Theoretical Aspects of Computer Science (STACS'09)

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    The Symposium on Theoretical Aspects of Computer Science (STACS) is held alternately in France and in Germany. The conference of February 26-28, 2009, held in Freiburg, is the 26th in this series. Previous meetings took place in Paris (1984), SaarbrĀØucken (1985), Orsay (1986), Passau (1987), Bordeaux (1988), Paderborn (1989), Rouen (1990), Hamburg (1991), Cachan (1992), WĀØurzburg (1993), Caen (1994), MĀØunchen (1995), Grenoble (1996), LĀØubeck (1997), Paris (1998), Trier (1999), Lille (2000), Dresden (2001), Antibes (2002), Berlin (2003), Montpellier (2004), Stuttgart (2005), Marseille (2006), Aachen (2007), and Bordeaux (2008). ..

    A Logic-Based Framework for Web Access Control Policies

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    With the widespread use of web services, there is a need for adequate security and privacy support to protect the sensitive information these services could provide. As a result, there has been a great interest in access control policy languages which accommodate large, open, distributed and heterogeneous environments like the Web. XACML has emerged as a popular access control language, but because of its rich expressiveness and informal semantics, it suffers from a) a lack of understanding of its formal properties, and b) a lack of automated, compile-time services that can detect errors in expressive, distributed and heterogeneous policies. In this dissertation, I present a logic-based framework for XACML that addresses the above issues. One component of the framework is a Datalog-based mapping for XACML v3.0 that provides a theoretical foundation for the language, namely: a concise logic-based semantics and complexity results for full XACML and various fragments. Additionally, my mapping discovers close relationships between XACML and other logic based languages such as the Flexible Authorization Framework. The second component of this framework provides a practical foundation for static analysis of expressive XACML policies. The analysis services detect semantic errors or differences between policies before they are deployed. To provide these services, I present a mapping from XACML to the Web Ontology Language (OWL), which is the standardized language for representing the semantics of information on the Web. In particular, I focus on the OWL-DL sub-language, which is a logic-based fragment of OWL. Finally, to demonstrate the practicality of using OWL-DL reasoners as policy analyzers, I have implemented an OWL-based XACML analyzer and performed extensive empirical evaluation using both real world and synthetic policy sets

    Proof-theoretic Semantics for Intuitionistic Multiplicative Linear Logic

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    This work is the first exploration of proof-theoretic semantics for a substructural logic. It focuses on the base-extension semantics (B-eS) for intuitionistic multiplicative linear logic (IMLL). The starting point is a review of Sandqvistā€™s B-eS for intuitionistic propositional logic (IPL), for which we propose an alternative treatment of conjunction that takes the form of the generalized elimination rule for the connective. The resulting semantics is shown to be sound and complete. This motivates our main contribution, a B-eS for IMLL , in which the definitions of the logical constants all take the form of their elimination rule and for which soundness and completeness are established

    Graphics Technology in Space Applications (GTSA 1989)

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    This document represents the proceedings of the Graphics Technology in Space Applications, which was held at NASA Lyndon B. Johnson Space Center on April 12 to 14, 1989 in Houston, Texas. The papers included in these proceedings were published in general as received from the authors with minimum modifications and editing. Information contained in the individual papers is not to be construed as being officially endorsed by NASA

    The 1992 4th NASA SERC Symposium on VLSI Design

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    Papers from the fourth annual NASA Symposium on VLSI Design, co-sponsored by the IEEE, are presented. Each year this symposium is organized by the NASA Space Engineering Research Center (SERC) at the University of Idaho and is held in conjunction with a quarterly meeting of the NASA Data System Technology Working Group (DSTWG). One task of the DSTWG is to develop new electronic technologies that will meet next generation electronic data system needs. The symposium provides insights into developments in VLSI and digital systems which can be used to increase data systems performance. The NASA SERC is proud to offer, at its fourth symposium on VLSI design, presentations by an outstanding set of individuals from national laboratories, the electronics industry, and universities. These speakers share insights into next generation advances that will serve as a basis for future VLSI design

    Excavations at the State House Inn Site, 18AP42, 15 State Circle, Annapolis, Maryland

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    During the spring of 1985, archaeological excavations were conducted at the State House Inn, 18AP42, 15 State Circle, Annapolis, Maryland. Work was conducted by "Archaeology in Annapolis," a cooperative project between Historic Annapolis, Inc. and the University of Maryland, College Park. This site is located within zone seven of the Maryland Archaeological Research Units (Figures 1, 2 & 3). A two-week program of testing in March, 1985 was carried out in the yard on State Circle. On the basis of positive results from this testing, six more weeks of excavations were carried out. This report summarizes the results of both phases of the excavations. Excavations were directed by Joseph W. Hopkins III, with the assistance of Donald Creveling and Paul Shackel. These excavations were part of a larger investigation of the Baroque town plan of Annapolis, laid out by Governor Francis Nicholson in 1695. This plan served as a framework around which the town grew over the next three centuries. Available historic records do not adequately document the development of the plan to its present form. The excavation program was a first step in a program to recover information about the gradual change of the city plan
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