Abstract. This paper presents an ontology-based approach for web querying, using semantic metadata. We propose a query language based on ontologies and emphasize its ability to express approximate queries, useful for an efficient information retrieval on the web. We present the Corese search engine dedicated to RDF(S) metadata and illustrate it through several real-world applications. 1

Simple conceptual graphs are considered as the kernel of most knowledge representation formalisms built upon $owa's model. Reasoning in this model can be expressed by a graph homomorphism called projection, whose semantics is usually given in terms of positive, conjunctive, existential FOL. We present here a family of extensions of this model, based on rules and constraints, keeping graph homomorphism as the basic operation. We focus on the formal definitions of the different models obtained, including their operational semantics and relationships with FOL, and we analyze the decidability and complexity of the associated problems (consistency and deduction). As soon as rules are involved in reasonings, these problems are not decidable, but we exhibit a condition under which they fall in the polynomial hierarchy. These results extend and complete the ones already published by the authors. Moreover we systematically study the complexity of some particular cases obtained by restricting the form of constraints and/or rules.

Abstract. In this paper, we aim at showing the advantages of Conceptual Graph formalism for the Semantic Web through several real-world applications in the framework of Corporate Semantic Webs. We describe the RDF(S)-dedicated semantic search engine, CORESE, based on a correspondence between RDF(S) and Conceptual Graphs, and we illustrate the interest of Conceptual Graphs through the analysis of several real-world applications based on CORESE. 1

this paper, we propose a meta-data representation model based on Tuple-Generating Dependencies. This representation uses concepts extracted from a thesaurus. In section 1 we state the problem of document retrieval, and present the Rameau thesaurus which defines the basic vocabulary used that can be used to build meta-data on documents. Depending on the use of meta-data for our application field, we propose in section 2 a classification in three levels. We formally describe in section 3 the meta-data model and inference rules from a logical point of view. Inference rules correspond to some "relevance rules". An inference mechanism for the implication problem allows the retrieval of documents that are relevant for the user. In section 4, we present the practical application of the model for document retrieval. This application uses the model of conceptual graphs as an interface between the formal framework of metadata and the user that may not be a computer scientist. We conclude with some future work and perspectives.

Abstract: The semantic web relies on ontologies representing domains through their main concepts and the relations between them. Such a domain knowledge is the keystone to represent the semantic contents of web resources and services in metadata associated to them. These metadata then enable us to search for information based on the semantics of web resources rather than their syntactic forms. However, in the context of the semantic web there are many possibilities of executing queries that would not retrieve any resource. The viewpoints of the designers of ontologies, of the designers of annotations and of the users performing a Web search may not completely match. The user may not completely share or understand the viewpoints of the designers and this mismatch may lead to missed answers. Approximate query processing is then of prime importance for efficiently searching the Semantic Web. In this paper we present the Corese ontology-based search engine we have developped to handle RDF(S) and OWL Lite metadata. We present its theoretical foundation, its query language, and we stress its ability to process approximate queries. Key-words: