Proceedings

example, in this sense, constitutes REA ontology (Resources-Events-Agents) proposed
by McCarthy in 1982 for the accounting field and described by an E-R diagram.
In our study, we have focused on particular OWL language created for the ontologies
development for the WEB. Available in version 2 (2009), the OWL represents a
declarative expression of the ontologies.
1.2. Web Ontology Language (OWL)
Web Ontology Language is developed by W3C (World Wide Web Consortium), an
international community. Specific OWL is the consideration of open worlds, i.e.
description of knowledge is not limited to a file or to a specific context. For example,
a class defined in ontology can be refined or extended in other ontologies. OWL
offers more syntax that can be used in various purposes: Functional-Style syntax,
RDF/XML syntax, RDF Turtle Syntax etc. Of these, in practice, the most used syntax
is RDF/XML which is based on the expressions defined in RDF (rdf:), RDFS (RDF
Schema - rdfs:) and XML Schema (xsd:).
The different concepts used OWL formalism in three categories: entities, expressions,
and axioms. Entities that constitute the foundation of structural ontologies are
represented by individuals, classes and properties. The elementary axioms are
statements relative to entities, and the expressions are built on the basis of complex
phrases entities. Individuals are concrete objects (instances), classes of domain
studied represent collections of individuals, and the attributes signify properties
(characteristics) of objects.
In OWL, a class can be defined in several ways: by specifying explicit its contents as
a subclass of existing classes (subClassOf), by applying logical operators
(intersectionOf, unionOf, complementOf, oneOf, etc.) on existing or undertake classes
by defining certain restrictions on properties of another class. Also, OWL offers two
predefined classes, Thing(T) and Nothing(⊥); the first stand at the basis of any
hierarchy, and the second can be designated as a subclass without instances for any
class in the hierarchy. The applied classes describe the axioms actually links between
these and may concern: a relationship of sub classing (subClassOf), a relationship in
which two classes have the same individuals (equivalentClass) or a relationship
through which individuals of a different classes are another class of individuals
(disjointWith).
Individuals are clearly defining the target dealing with classes that they belong to.
Axioms applied to individuals are sameIndividualAs (two individuals covered by the
same concept) and differentFrom (previous deny).
Properties in OWL can be of type object (ObjectProperty), stating that correlate
individuals of two classes or type of date (DataTypeProperty), involving elementary
values (integer, string, etc). Regardless of their type, the properties may be
characterized by a set of axioms: subPropertyOf (property is a specialization of
another), InverseOf (the two properties are additional/opposite), functionalProperty
(describes a property mono-value), and transitiveProperty (if property P has connects
the object A to B and B to C, then P tie on A and C) etc.
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