[Semantic Web]Axioms in OWL

读书笔记,  科学研究 Lee 发表于 2006/2/23 12:54:03 There is three kind of axioms in OWL.   1.Class Axioms   Class descriptions form the building blocks for defining classes through class axioms. The simplest form of a class axiom is a class description of type 1, It just states the existence of a class, using owl:Class with a class identifier.  This is correct OWL, but does not tell us very much about the class Human. Class axioms typically contain additional components that state necessary and/or sufficient characteristics of a class. OWL contains three language constructs for combining class descriptions into class axioms:  rdfs:subClassOf allows one to say that the class extension of a class description is a subset of the class extension of another class description.  owl:equivalentClass allows one to say that a class description has exactly the same class extension as another class description.  owl:disjointWith allows one to say that the class extension of a class description has no members in common with the class extension of another class description.  Syntactically, these three language constructs are properties that have a class description as both domain and range. We discuss these properties in more detail in the following subsections.  In addition, OWL allows users to define class axioms by giving a name to class descriptions of the enumeration or set-operator type. Such a class axiom defines necessary and sufficient conditions for establishing class membership.    2. Property Axioms   Properties are specified using a frame-like syntax. Data-valued properties relate individuals to data values, like integers. Individual-valued properties relate individuals to other individuals. These two kinds of properties can be given super-properties, allowing the construction of a property hierarchy. It does not make sense to have an individual-valued property be a super-property of a data-valued property, or vice versa. Data-valued and individual-valued properties can also be given domains and ranges. A domain for a property specifies which individuals are potential subjects of statements that have the property as predicate, just as in RDFS. In OWL Lite the domains of properties are classes. There can be multiple domains, in which case only individuals that belong to all of the domains are potential subjects. A range for a property specifies which individuals or data values can be objects of statements that have the property as predicate. Again, there can be multiple ranges, in which case only individuals or data values that belong to all of the ranges are potential objects. In OWL Lite ranges for individual-valued properties are classes; ranges for data-valued properties are datatypes.  Data-valued properties can be specified as (partial) functional, i.e., given an individual, there can be at most one relationship to a data value for that individual in the property. Individual-valued properties can be specified to be the inverse of another property.  Individual-valued properties can also be specified to be symmetric as well as partial functional, partial inverse-functional, or transitive.  To preserve decidability of reasoning in OWL Lite, not all properties can have cardinality restrictions placed on them or be specified as functional or inverse-functional. An individual-valued property is complex if 1/ it is specified as being functional or inverse-functional, 2/ there is some cardinality restriction that uses it, 3/ it has an inverse that is complex, or 4/ it has a super-property that is complex. Complex properties cannot be specified as being transitive.  Annotation and ontology properties are much simpler than data-valued and individual-valued properties. The only information in axioms for them is annotations.  Property axioms in the OWL DL abstract syntax generalize OWL Lite property axioms by allowing descriptions in place of classes and data ranges in place of datatypes in domains and ranges.    3.Restrictions   Restrictions are used in OWL Lite class axioms to provide local constraints on properties in the class. Each allValuesFrom part of a restriction makes the constraint that all values of the property for individuals in the class must belong to the specified class or datatype. Each someValuesFrom part makes the constraint that there must be at least one value for the property that belongs to the specified class or datatype. The cardinality part says how many distinct values there are for the property for each individual in the class. In OWL Lite the only cardinalities allowed are 0 and 1.  Restrictions in the OWL DL abstract syntax generalize OWL Lite restrictions by allowing descriptions where classes are allowed in OWL Lite and allowing sets of data values as well as datatypes. The combination of datatypes and sets of data values is called a data range. In the OWL DL abstract syntax, values can also be given for properties in classes. In addition, cardinalities are not restricted to only 0 and 1.                                                       ----from OWL Specifications

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