Research on ontology is becoming increasingly widespread in the computer science community, and its importance is being recognized in a multiplicity of research fields and application areas, including knowledge engineering, database design and integration, information retrieval and extraction. We shall use the generic term information systems, in its broadest sense, to collectively refer to these application perspectives. We argue in this paper that so-called ontologies present their own methodological and architectural peculiarities: on the methodological side, their main peculiarity is the adoption of a highly interdisciplinary approach, while on the architectural side the most interesting aspect is the centrality of the role they can play in an information system, leading to the perspective of ontology-driven information systems.

Taxonomies are an important part of conceptual modeling. They provide substantial structural information, and are typically the key elements in integration efforts, however there has been little guidance as to what makes a proper taxonomy. We have adopted several notions from the philosophical practice of formal ontology, and adapted them for use in information systems. These tools, identity, essence, unity, and dependence, provide a solid logical framework within which the properties that form a taxonomy can be analyzed. This analysis helps make intended meaning more explicit, improving human understanding and reducing the cost of integration.

The purpose of this paper is to explore some semantic problems related to the use of linguistic ontologies in information systems, and to suggest some organizing principles aimed t o solve such problems. The taxonomic structure of current ontologies is unfortunately quite complicated and hard to understand, especially for what concerns the upper levels. I will focus here on the problem of ISA overloading, which I believe is the main responsible of these difficulties. To this purpose, I will carefully analyze the ontological nature of the categories used in current upper-level structures, considering the necessity of splitting them according to more subtle distinctions or the opportunity of excluding them because of their limited organizational role.

The task of information extraction can be seen as a problem of semantic matching between a user-defined template and a piece of information written in natural language. To this purpose, the ontological assumptions of the template need to be suitably specified, and compared with the ontological implications of the text. So-called "ontologies", consisting of theories of various kinds expressing the meaning of shared vocabularies, begin to be used for this task. This paper addresses the theoretical issues related to the design and use of such ontologies for purposes of information retrieval and extraction. After a discussion on the nature of semantic matching within a model-theoretical framework, we introduce the subject of Formal Ontology, showing how the notions of parthood, integrity, identity, and dependence can be of help in understanding, organizing and formalizing fundamental ontological distinctions. We present then some basic principles for ontology design, and we illustrate a preliminary proposal for a top-level ontology develped according to such principles. As a concrete example of ontology-based information retrieval, we finally report an ongoing experience of use of a large linguistic ontology for the retrieval of object-oriented software components.

Recent proposals to improve the quality of interaction with the World Wide Web suggest considering the Web as a huge semistructured database, so that retrieving information can be supported by the task of database querying. Under this view, it is important to represent the form of both the network, and the documents placed in the nodes of the network. However, the current proposals do not pay sufficient attention to represent document structures and reasoning about them. In this paper, we address these problems by providing a framework where Document Type Definitions (DTDs) expressed in the eXtensible Markup Language (XML) are formalized in an expressive Description Logic equipped with sound and complete inference algorithms. We provide methods for verifying conformance of a document to a DTD in polynomial time, and structural equivalence of DTDs in worst case deterministic exponential time, improving known algorithms for this problem which were double exponential. We also deal with parametric versions of conformance and structural equivalence, and investigate other forms of reasoning on DTDs. Finally, we show how to take advantage of the reasoning capabilities of our formalism in order to perform several optimization steps in answering queries posed to a document base.

For the basic Description Logics reasoning with respect to finite models amounts to reasoning with respect to arbitrary ones, but finiteness of the domain needs to be considered if expressivity is increased and the finite model property fails. Procedures for reasoning with respect to arbitrary models in very expressive Description Logics have been developed, but these are not directly applicable in the finite case. We first show that we can nevertheless capture a restricted form of finiteness and represent finite modeling structures such as lists and trees, while still reasoning with respect to arbitrary models. The main result of this paper is a procedure to reason with respect to finite models in an expressive Description Logic equipped with inverse roles, cardinality constraints, and in which arbitrary inclusions between concepts can be specified without any restriction. This provides the necessary expressivity to go beyond most semantic and object-oriented Database models, and capture several useful extensions. 1

. Aggregated objects play an important role in many knowledge representation applications. For the adequate representation of aggregated objects, it is crucial to represent part-whole relations. We discuss properties of part-whole relations and extend the description logic ALC with means for the adequate representation of part-whole relations and thus of aggregated objects. 1 Motivation Description logics are a family of knowledge representation formalisms well-suited for the representation of and reasoning about configurations [27, 21], ontologies [19], and database schemata, where they can support schema design, evolution, and query optimisation [4, 7], source integration in heterogeneous databases/data warehouses [5, 6], and conceptual modeling of multidimensional aggregation [11]. In all these applications, aggregated objects play a central role, that is, objects that are composed of various parts, which again can be composite, etc. It is natural to describe an aggregated object...

In a conventional information mediation scenario it is assumed that all sources, including their schemas, are known before the integrated view is defined.

Concept models, or ‘ontologies’ represented in description logics are becoming central to many efforts in re-usable medical terminologies. Developers of any ontology must make a series of choices concerning how concepts are represented. These choices are made more difficult by the limitations of the logical formalisms in which ontologies are represented and the complexity of the pragmatics of medical usage, as well as by the sheer size and scope of medicine. Principles for ontology structure to guide developers are still poorly. This paper articulates the principles arrived at from the experience of the GALEN programme and illustrates them with an overview of the its ontology, the GALEN Common Reference Model. The complete Common Reference Model and associated material is available from the OpenGALEN foundation at www.OpenGALEN.org

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