Towards Knowledge Discovery in the Semantic Web
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The Resource Description Framework (RDF) (Klyne et al., 2004) is a framework for representing information on the Web. RDF allows anyone to make statements about any resource, which could be a material or immaterial thing. A statement is defined as a triple, consisting a subject s, predicate p and object o, written as p(s,o). This means that s has a predicate (or property) p with value o. RDF is based on a graph data model. A RDF graph G=(V,E) is a representation of the document triples. A node n of the graph could be a subject s or object o, which is connected through a directed arc (s,o) that represents the predicate p. RDF-Schema (Brickley et al., 2004) is a minimal ontological language. It has capabilities to define classes and properties, and enables the specification of how they should be used together. Classes and properties could be arranged in a hierarchy. Instances of a class are referenced to its class through the ”rdf:type” definition. RDF-Schema provides means to define a simple shared vocabulary. Nevertheless, its expressiveness is limited. Amongst others things, it provides no support for cardinality constraints on properties, transitive properties as well as equivalence and disjointness relationships of classes and individuals. The Web Ontology Language (OWL) is more expressive than RDF-Schema and is thus considered below in more detail.
Mining-State of the Art and Future Directions. Journal of Web Semantics,