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Ontologies are used to capture knowledge about some domain of interest. ◦ An ontology describes the concepts in the domain and also the relationships that hold between those concepts. Standard ontology languages is OWL from the World Wide Web Consortium (W3C)
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Complex concepts be built up in definitions out of simpler concepts. The logical model allows the use of a reasoner which can check whether or not all of the statements and definitions in the ontology are mutually consistent and can also recognize which concept fit under which definitions.
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Individuals (instances) Properties(slots in Protégé, relations in UML, attributes) Classes(concepts)
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Individuals, represent objects in the domain in which we are interested.
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Properties are binary relations on individuals
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OWL classes are interpreted as sets that contain individuals. Classes may be organized into a superclass-subclass hierarchy (taxonomy).
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classes are disjoint, so that an individual (or object) cannot be an instance of more than one of these classes.
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Figure 4.10: Class Hierarchy
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Object properties Datatype properties Annotation properties
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Inverse Property: ◦ Each object property may have a corresponding inverse property. If some property links individual a to individual b then its inverse property will link individual b to individual a.
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Functional Properties(single valued properties, features) ◦ If a property is functional, for a given individual, there can be at most one individual that is related to the individual via the property.
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Inverse Functional Properties ◦ If a property is inverse functional then it means that the inverse property is functional.
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Transitive Properties ◦ If a property is transitive, and the property relates individual a to individual b, and also individual b to individual c, then we can infer that individual a is related to individual c via property P.
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Symmetric Properties ◦ If a property P is symmetric, and the property relates individual a to individual b then individual b is also related to individual a via property P.
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Asymmetric properties ◦ If a property P is asymmetric, and the property relates individual a to individual b then individual b cannot be related to individual a via property P.
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Reflexive properties ◦ A property P is said to be reflexive when the property must relate individual l to itself.
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Irreflexive properties If a property P is irreflexive, it can be described as a property that relates an individual a to individual b,where individual a and individual b are not the same.
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Properties may have a domain and a range specified. Properties link individuals from the domain to individuals from the range. It is important to realize that in OWL domains and ranges should not be viewed as constraints to be checked. They are used as `axioms' in reasoning.
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Property Restrictions Existential (∃) and Universal Restrictions (∀)
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Reasoners are sometimes called classifiers. Classification, however, is not the only inference service provided by reasoners. For instance, a reasoner also performs consistency checking which is different from classification. Hence, the preferred term is reasoner. Protégé 4 allows different OWL reasoners to be plugged in, the reasoner shipped with Protégé is called Fact++. The ontology can be `sent to the reasoner' to automatically compute the classification hierarchy, and also to check the logical consistency of the ontology.
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Figure 4.42: The Class ProbeInconsistentTopping found to be inconsistent by the reasoner
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Necessary conditions can be read as, If something is a member of this class then it is necessary to fulfill these conditions". If class A is described using necessary conditions, then we can say that if an individual is a member of class A it must satisfy the conditions. We cannot say that any (random) individual that satisfies these conditions must be a member of class A. However, if class A is now defined using necessary and sufficient conditions, we can say that if an individual is a member of the class A it must satisfy the conditions and we can now say that if any (random) individual satisfies these conditions then it must be a member of class A. Necessary conditions are simply called Superclasses in Protégé 4. Necessary and sufficient condition are called Equivalent classes.
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For example, we might dene a class DaysOfTheWeek to contain the individuals (and only the individuals) Sunday, Monday, Tuesday, Wednesday, Thursday, Friday and Saturday.
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To restrict the relationships for a given property to individuals that are members of a specific class we must use a universal restriction. Universal restrictions are given the ∀. They constrain the relationships along a given property to individuals that are members of a specific class.
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A closure axiom on a property consists of a universal restriction that acts along the property to say that it can only be filled by the specified fillers. For example, the closure axiom on the hasTopping property for MargheritaPizza is a universal restriction that acts along the hasTopping property, with a filler that is the union of MozzarellaTopping and also TomatoTopping.
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Figure 4.51: Class Description View: Margherita Pizza With a Closure Axiom for the hasTopping property
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A covering axiom consists of two parts:The class that is being `covered', and the classes that form the covering.
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For a given property P, a Minimum Cardinality Restriction species the minimum number of P relationships that an individual must participate in. A Maximum Cardinality Restriction specifies the maximum number of P relationships that an individual can participate in.
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Datatype properties link an individual to an XML Schema Datatype value or an rdf literal. In other words, they describe relationships between an individual and data values.
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OWL allows classes, properties, individuals and the ontology itself (technically speaking the ontology header) to be annotated with various pieces of information/meta-data. These pieces of information may take the form of auditing or editorial information.
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Create Pizza Ontology by Protégé.
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