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ISIT315: Semantic Web > 05: RDF Continued > Flashcards

05: RDF Continued Flashcards

1
Q

Basic Ideas of RDF Schema

A
  • RDF is a universal language that lets users describe resources in their own vocabularies by using:
    • Classes and properties
    • Class hierarchies and inheritance
    • Property hierarchies
  • Does not assume/define semantics
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2
Q

Instances

A
  • Individual objects that belong to a class are instances of that class
  • Relationship between instances and classes in RDF is though rdf:type
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3
Q

Why are classes useful?

A
  • Classes impose restrictions on what can be stated in an RDF document using the schema
    • Disallows nonsense
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4
Q

Class Hierarchies

A
  • A is a subclass of B if every instance of A is also an instance of B
    • Therefore, B is a superclass ​of A
  • A subclass graph need not be a tree
  • A class may have multiple superclasses
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5
Q

Inheritance in Class Hierarchies

A
  • Holds a range restriction, i.e. Courses must be taught by academic staff members only

“Michael Maher is a professor.”

  • Michael Maher inherits the ability to teach from the class of academic staff members
  • This is achieved in an RDF schema by fixing the semantics of “is a subclass of”
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6
Q

Hierarchical Relationships for Properties

A

e.g. “is taught by” is a sub-property of “involves”

  • If a course C is taught by an academic staff member A, then C also involves A
    • Converse is not necessarily true
  • P is a sub-property of Q, if Q(x,y) is true whenever P(x,y) is true.
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7
Q

RDF Layer vs RDF Schema Layer

A
  • Schema is written in a formal language using expressions like subClassOf, Class, Property, subPropertyOf, Resource, etc.
  • Think: relational database schema (RDF Schema Layer) vs. actual instances of class relations (RDF Layer)
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8
Q

The modeling primitives of RDF Schema are defined using _______ and _______.

A

The modeling primitives of RDF Schema are defined using resources and properties.

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9
Q

Core Classes

A
  • rdfs:Resources, class of all resources
  • rdfs:Class, class of all classes
  • rdfs:Literal, class of all literals (strings)
  • rdfs:Property, class of all properties
  • rdfs:Statement, class of all reified statements
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10
Q

Core Properties

A
  • rdf:type, which relates a resource to its class (resource is declared to be an instance of that class)
  • rdfs:subClassOf, which relates a class to one of its superclasses (all instances of a class are instances of its superclass)
  • rdfs:subPropertyOf, elates a property to one of its superproperties
  • rdfs:domain, which specifies the domain of a property P
    • Class of those resources that may appear as subjects in a triple with predicate P
    • If the domain is not specified, then any resource can be the subject
  • rdfs:range, which specifies the range of a property P
    • Class of those resources that may appear as values in a triple with predicate P
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11
Q

rdfs:subClassOf and rdfs:subPropertyOf are _____, by definition.

A

rdfs:subClassOf and rdfs:subPropertyOf are transitive, by definition.

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12
Q

Reification and Containers

A
  • rdf:subject, relates a reified statement to its subject
  • rdf:predicate, relates a reified statement to its predicate
  • rdf:object, relates a reified statement to its object
  • rdf:Bag, the class of bags
  • rdf:Seq, the class of sequences
  • rdf:Alt, the class of alternatives
  • rdfs:Container, which is a superclass of all container classes, including the three above
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13
Q

Utility Properties

A
  • rdfs:seeAlso relates a resource to another resource that explains it
  • rdfs:isDefinedBy is a subproperty of rdfs:seeAlso and relates a resource to the place where its definition, typically an RDF schema.
  • rdfs:comment. Comments, typically longer text, can be associated with a resource
  • rdfs:label. A human-friendly label (name) is associated with a resource
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14
Q

Three Purposes of Axiomatic Semantics

A
  1. Formalizing the meaning of the modeling primitives of RDF and RDF Schema by translating them into first-order logic
  2. Making the semantics unambiguous and machine accessible
  3. Providing a basis for reasoning support by automated reasoners manipulating logical formulas
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15
Q

First Order Logic

A
  • AKA predicate logic
  • Collection of formal systems used in mathematics, philosophy, linguistics, and computer science
  • Uses quantified variables over non-logical objects and allows the use of sentences that contain variables
  • There are predicates having predicates or functions as arguments, or in which one or both of predicate quantifiers or function quantifiers are permitted
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16
Q

Function Symbols

(An Auxiliary Axiomatisation of Lists)

A
  • nil - empty list
  • cons(x,l) - adds an element to the front of the list
  • first(l) - returns the first element
  • rest(l) - returns the rest of the list
17
Q

Predicate Symbols

(An Auxiliary Axiomatisation of Lists)

A
  • item(x,l) - tests if an element occurs in the list
  • list(l) - tests whether l is a list
18
Q

Basic Predicates

A
  • PropVal(P,R,V)
    • A predicate with 3 arguments, which is used to represent an RDF statement with resource R, property P and value V
    • An RDF statement (triple) (P,R,V) is represented as PropVal(P,R,V)
  • Type(R,T)
    • Short for PropVal(type,R,T)
    • Specifies that the resource R has the type T
  • Type(?r,?t) <–> PropVal(type,?r,?t)
19
Q

Biconditional

A

if and only if; iff

A <–> B

e.g. either both A and B are correct or both are wrong

20
Q

Implication

A

if, then

A –> B

e.g. if A is correct, then B is correct

21
Q

By using the implication symbol, write an implication formula for the following statement in the image.

(Hint: complete the following PropVal(domain,?p,?d) –> ???)

A
22
Q

By using the implication symbol, write an implication formula for the following statement in the image.

(Hint: complete the following PropVal(range,?p,?r) –> ???)

A
23
Q

By using the biconditional symbol, write an implication formula for the following statement in the image.

(Hint: complete the following PropVal(subPropertyOf,?p,?p’) ???)

A
24
Q

Semantics Based on Inference Rules

A
  • Semantics in terms of RDF Triples instead of restating RDF in terms of first-order logic
    • ​Sound and complete inference systems
25
Q

Inference System Rules

A

IF E contains certain triples, THEN add to E certain additional triples

(where E is an arbitrary set of RDF triples)

ISIT315: Semantic Web (14 decks)

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