07: Web Ontology Language: OWL Flashcards
A reasoner expands the ____ ____ ____ based on ____ like rdfs:subClassOf, rdfs:range, rdfs:domain, etc.
A reasoner expands the number of triples based on relations like rdfs:subClassOf, rdfs:range, rdfs:domain, etc.
Five Requirements of Any Ontology Language
- A well-defined syntax
- Efficient reasoning support
- A formal semantics
- Sufficient expressive power
- Convenience of expression
What is the tradeoff between expressive power and efficient reasoning support? What compromise must be taken?
the richer the language, the more inefficient the reasoning support
Compromise:
- on one hand, a language supported by reasonable efficient reasoners
- on the other hand, a language that can express large classes of ontologies and knowledge
Class Membership
(Reasoning About Knowledge in Ontology Languages)
If x is an instance of a class C, and C is a subclass of D, then we can infer that x is an instance of D
Equivalence of Classes
(Reasoning About Knowledge in Ontology Languages)
If class A is equivalent to class B, and class B is equivalent to class C, then A is equivalent to C, too
Consistency
(Reasoning About Knowledge in Ontology Languages)
X instance of classes A and B, but A and B are disjoint
* this is an indication of an error in the ontology
Classification
(Reasoning About Knowledge in Ontology Languages)
Certain property-value pairs are a sufficient condition for membership in a class A; if an individual x satisfies such conditions, we can conclude that x must be an instance of A
Uses for Reasoning
Reasoning support is important for:
- checking the consistency of the ontology and the knowledge
- checking for unintended relationships between classes
- automatically classifying instances in classes
Checks like the preceding ones are valuable for:
- designing large ontologies, where multiple authors are involved
- integrating and sharing ontologies from various sources
Reasoning Support for OWL
- Semantics is a prerequisite for reasoning support
-
Formal semantics and reasoning support are usually provided by
- mapping an ontology language to a known logical formalism
- using automated reasoners that already exist for those formalisms
OWL is (partially) mapped on a _____ logic, and makes use of reasoners such as ____ and ____.
OWL is (partially) mapped on a description logic, and makes use of reasoners such as FaCT and RACER.
Combining RDF Schema with ____ leads to uncontrollable ____ properties.
Combining RDF Schema with logic leads to uncontrollable computational properties.
The Five Major Limitations of the Expressive Power of RDF Schema in Comparison with OWL
- Local scope of properties
- e.g. cannot declare range restrictions that apply to some classes only like a cow can only eat plants, but other animals can eat meat too
- Disjointness of classes
- e.g. cannot disjoint classes, like graduate vs undergraduate
- Boolean combinations of classes
- e.g. cannot combine classes using union, intersection, complement
- Cardinality restrictions
- e.g. cannot state a person has exactly two parents, or a course has exactly one lecturer
- Special characteristics of properties
- e.g. cannot state transitive, unique, or inverse properties
Three Species of OWL
- OWL Full
- OWL DL
- OWL Lite
OWL Full
(Three Species of OWL)
- Uses all the OWL languages primitives
- Allows the combination of these primitives in arbitrary ways with RDF and RDF Schema
- Fully upward-compatible with RDF, both syntactically and semantically
- So powerful that it is undecidable
- No complete (or efficient) reasoning support
OWL DL
(Three Species of OWL)
- Sublanguage of OWL Full that restricts application of the constructors from OWL and RDF
- Application of OWL’s constructors to each other is disallowed
- Therefore it corresponds to a well studied description logic
- Permits efficient reasoning support
-
But we lose full compatibility with RDF:
- Not every RDF document is a legal OWL DL document.
- Every legal OWL DL document is a legal RDF document.
OWL Lite
(Three Species of OWL)
- An even further restriction limits OWL DL to a subset of the language constructors
- e.g. OWL Lite excludes enumerated classes, disjointness statements, and arbitrary cardinality.
- The advantages of this is a language that is easier to
- grasp, for users
- implement, for tool builders
- The disadvantage is restricted expressivity
Upward Compatibility between OWL Species
- Legal OWL Lite ontology = Legal OWL DL ontology
- Legal OWL DL ontology = Legal OWL Full ontology
- Valid OWL Lite conclusion = Valid OWL DL conclusion
- Valid OWL DL conclusion = Valid OWL Full conclusion
Semantic Web design aims at ____ ____ with corresponding reuse of software across the various layers
Semantic Web design aims at downward** **compatibility with corresponding reuse of software across the various layers
The advantage of full downward compatibility for OWL is only achieved for ____ ____, at the cost of ____ ____
The advantage of full downward compatibility for OWL is only achieved for OWL** **Full**, at the cost of **computational intractability
OWL Syntactic Varieties
Other syntactic forms for OWL have also been defined:
- An alternative, more readable XML-based syntax
- An abstract syntax, that is much more compact and readable than the XML languages
- A graphic syntax based on the conventions of UML
Write the syntax of an OWL XML/RDF header.
owl:imports is a ____ property
owl:imports is a transitive property
Classes are defined using ____
(OWL RDF/XML Syntax)
Classes are defined using owl:Class
Disjointness is defined using ____
(OWL RDF/XML Syntax)
Disjointness is defined using owl:disjointWith
____ defines equivalence of classes
(OWL RDF/XML Syntax)
owl:equivalentClass defines equivalence of classes
____ is the most general class, which contains everything
(OWL RDF/XML Syntax)
owl:Thing is the most general class, which contains everything
____ is the empty class
(OWL RDF/XML Syntax)
owl:Nothing is the empty class
The Two Properties of OWL
-
Object properties, which relate objects to other objects
- e.g. is-TaughtBy, supervises
-
Data type properties, which relate objects to datatype values
- e.g. phone, title, age, etc.
Data Type Properties
(The Two Properties of OWL)
OWL makes use of XML Schema data types, using the layered architecture of the Semantic Web
Object Properties
(The Two Properties of OWL)
user-defined data types
____ defines inverse properties
(OWL RDF/XML Syntax)
owl:inverseOf defines inverse properties
____ defines equivalent properties
(OWL RDF/XML Syntax)
owl:equivalentProperty defines equivalent properties
Property Restrictions
- A (restriction) class is achieved through an owl:Restriction element
- This element contains an owl:onProperty element and one or more restriction declarations
- One type defines cardinality restrictions
- The other type defines restrictions on the kinds of values the property may take
-
owl:allValuesFrom specifies universal quantification
- ∀ (the universal quantifier symbol)
- owl:hasValue specifies a specific value
-
owl:someValuesFrom specifies existential quantification
- ∃ (read: “there exists”).
-
owl:allValuesFrom specifies universal quantification
True or False: An owl:Restriction element can contain only one restriction declaration
False
* can contain one or more restriction declarations
Write the equivalence of the following in English.
Only professors teach first-year subjects.
Write the equivalence of the following in English.
One of the math courses is taught by #949352.
Write the equivalence of the following in English.
Some tutors teach undergraduate courses.
What element do you use to specify a minimum and maximum number?
owl:minCardinality and owl:maxCardinality
How do you specify a precise number by using cardinality restrictions?
Use the same minimum and maximum number
Special Properties
-
owl:TransitiveProperty (transitive property)
- e.g. “has better grade than”, “is ancestor of”
-
owl:SymmetricProperty (symmetry)
- e.g. “has same grade as”, “is sibling of”
-
owl:FunctionalProperty defines a property that has at most one value for each object
- e.g. “age”, “height”, “directSupervisor”
- owl:InverseFunctionalProperty defines a property for which two different objects cannot have the same value
Write three examples for each of transitive and symmetric properties.
SYMMETRIC:
- “X isMarriedTo Y” means “Y isMarriedTo X”.
- “Z equals D“ means “D equals Z“
- ” A co-worker-of B” means “ B co-worker-of A”
TRANSITIVE:
- If A implies B and B implies C, then A implies C
- If “Z equals D” and “D equals E” means “Z equals E“
- If “X sub-set-of Y” and “Y sub-set-of Z” means “X sub-set Z“
The ____ construct selects all individuals from the domain of discourse that do not belong to a certain ____. Usually, this refers to a very large set of individuals.
The complementOf construct selects all individuals from the domain of discourse that do not belong to a certain class. Usually, this refers to a very large set of individuals.
Assuming #staffMember, #faculty (green), and #techSupportStaff” (blue) have all been defined as classes, by the use of nesting Boolean operators, complete the following code for defining adminStaff (red).
Hint:
#staffMember = #adminStaff + #faculty + #techSupportStaff
