07: Web Ontology Language: OWL

Question 1

A reasoner expands the ____ ____ ____ based on ____ like rdfs:subClassOf, rdfs:range, rdfs:domain, etc.

Answer 1

A reasoner expands the number of triples based on relations like rdfs:subClassOf, rdfs:range, rdfs:domain, etc.

Question 2

Five Requirements of Any Ontology Language

Answer 2

1. A well-defined syntax
2. Efficient reasoning support
3. A formal semantics
4. Sufficient expressive power
5. Convenience of expression

Question 3

What is the tradeoff between expressive power and efficient reasoning support? What compromise must be taken?

Answer 3

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

Question 4

Class Membership

(Reasoning About Knowledge in Ontology Languages)

Answer 4

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

Question 5

Equivalence of Classes

(Reasoning About Knowledge in Ontology Languages)

Answer 5

If class A is equivalent to class B, and class B is equivalent to class C, then A is equivalent to C, too

Question 6

Consistency

(Reasoning About Knowledge in Ontology Languages)

Answer 6

X instance of classes A and B, but A and B are disjoint

* this is an indication of an error in the ontology

Question 7

Classification

(Reasoning About Knowledge in Ontology Languages)

Answer 7

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

Question 8

Uses for Reasoning

Answer 8

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

Question 9

Reasoning Support for OWL

Answer 9

• 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

Question 10

OWL is (partially) mapped on a _____ logic, and makes use of reasoners such as ____ and ____.

Answer 10

OWL is (partially) mapped on a description logic, and makes use of reasoners such as FaCT and RACER.

Question 11

Combining RDF Schema with ____ leads to uncontrollable ____ properties.

Answer 11

Combining RDF Schema with logic leads to uncontrollable computational properties.

Question 12

The Five Major Limitations of the Expressive Power of RDF Schema in Comparison with OWL

Answer 12

1. 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
2. Disjointness of classes
   
   • ​​e.g. cannot disjoint classes, like graduate vs undergraduate
3. Boolean combinations of classes
   
   • ​​e.g. cannot combine classes using union, intersection, complement
4. Cardinality restrictions
   
   • ​​e.g. cannot state a person has exactly two parents, or a course has exactly one lecturer
5. Special characteristics of properties
   
   • ​​e.g. cannot state transitive, unique, or inverse properties

Question 13

Three Species of OWL

Answer 13

1. OWL Full
2. OWL DL
3. OWL Lite

Question 14

OWL Full

(Three Species of OWL)

Answer 14

• 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

Question 15

OWL DL

(Three Species of OWL)

Answer 15

• 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.

Question 16

OWL Lite

(Three Species of OWL)

Answer 16

• 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

Question 17

Upward Compatibility between OWL Species

Answer 17

• 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

Question 18

Semantic Web design aims at ____ ____ with corresponding reuse of software across the various layers

Answer 18

Semantic Web design aims at downward** **compatibility with corresponding reuse of software across the various layers

Question 19

The advantage of full downward compatibility for OWL is only achieved for ____ ____, at the cost of ____ ____

Answer 19

The advantage of full downward compatibility for OWL is only achieved for OWL** **Full**, at the cost of **computational intractability

Question 20

OWL Syntactic Varieties​

Answer 20

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

Question 21

Write the syntax of an OWL XML/RDF header.

Answer 21

Question 22

owl:imports is a ____ property

Answer 22

owl:imports is a transitive property

Question 23

Classes are defined using ____

(OWL RDF/XML Syntax)

Answer 23

Classes are defined using owl:Class

Question 24

Disjointness is defined using ____

(OWL RDF/XML Syntax)

Answer 24

Disjointness is defined using owl:disjointWith

Question 25

**\_\_\_\_ defines equivalence of classes** ## Footnote **(OWL RDF/XML Syntax)**

Answer 25

**_owl:equivalentClass_** defines equivalence of classes

Question 26

**\_\_\_\_ is the most general class, which contains everything** ## Footnote **(OWL RDF/XML Syntax)**

Answer 26

**_owl:Thing_** is the most general class, which contains everything

Question 27

**\_\_\_\_ is the empty class** ## Footnote **(OWL RDF/XML Syntax)**

Answer 27

**_owl:Nothing_** is the empty class

Question 28

**The Two Properties of OWL**

Answer 28

* **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.

Question 29

**Data Type Properties** ## Footnote **(The Two Properties of OWL)**

Answer 29

OWL makes use of XML Schema data types, using the layered architecture of the Semantic Web

Question 30

**Object Properties** ## Footnote **(The Two Properties of OWL)**

Answer 30

user-defined data types

Question 31

**\_\_\_\_ defines inverse properties** ## Footnote **(OWL RDF/XML Syntax)**

Answer 31

**_owl:inverseOf_** defines inverse properties

Question 32

**\_\_\_\_ defines equivalent properties** ## Footnote **(OWL RDF/XML Syntax)**

Answer 32

**_owl:equivalentProperty_** defines equivalent properties

Question 33

**Property Restrictions**

Answer 33

* 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”).

Question 34

**True or False: An owl:Restriction element can contain only one restriction declaration**

Answer 34

**False** ## Footnote \* can contain one or more restriction declarations

Question 35

**Write the equivalence of the following in English.**

Answer 35

**Only professors teach first-year subjects.**

Question 36

**Write the equivalence of the following in English.**

Answer 36

**One of the math courses is taught by #949352.**

Question 37

**Write the equivalence of the following in English.**

Answer 37

**Some tutors teach undergraduate courses.**

Question 38

**What element do you use to specify a minimum and maximum number?**

Answer 38

**owl:minCardinality** and **owl:maxCardinality**

Question 39

**How do you specify a precise number by using cardinality restrictions?**

Answer 39

**Use the same minimum and maximum number**

Question 40

**Special Properties**

Answer 40

* **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

Question 41

**Write three examples for each of transitive and symmetric properties.**

Answer 41

**SYMMETRIC:** 1. "X isMarriedTo Y" means "Y isMarriedTo X". 2. "Z equals D“ means “D equals Z“ 3. " A co-worker-of B" means " B co-worker-of A" **TRANSITIVE:** 1. If A implies B and B implies C, then A implies C 2. If "Z equals D" and "D equals E" means “Z equals E“ 3. If “X sub-set-of Y" and “Y sub-set-of Z" means “X sub-set Z“

Question 42

**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.**

Answer 42

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.

Question 43

**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**

Answer 43