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Dr. Harald Sack
Hasso Plattner Institute for IT Systems Engineering
University of Potsdam
Spring 2013
Semantic Web Technologies
Lecture 4: Knowledge Representations I02: Ontologies in Computer Science
Semantic Web Technologies , Dr. Harald Sack, Hasso-Plattner-Institut, Universität Potsdam
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Lecture 4: Knowledge Representations I
Open HPI - Course: Semantic Web Technologies
Semantic Web Technologies , Dr. Harald Sack, Hasso-Plattner-Institut, Universität Potsdam
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02 Ontologies in Computer Science Open HPI - Course: Semantic Web Technologies - Lecture 4: Knowledge Representations I
Semantic Web Technologies , Dr. Harald Sack, Hasso-Plattner-Institut, Universität Potsdam
416 "An ontology is an explicit, formal specification of a shared conceptualization. The term is borrowed from philosophy, where an Ontology is a systematic account of Existence. For AI systems, what ‘exists’ is that which can be represented.“
Thomas R. Gruber: A Translation Approach to Portable Ontology Specifications.Knowledge Acquisition, 5(2):199-220, 1993.
conceptualization: abstract model (domain, identified relevant concepts, relations)
explicit: meaning of all concepts must be defined
formal: machine understandable
shared: consensus about ontology
tomgruber.org
The Concept of Ontology in Computer Science
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Conceptualization
Symbol Object
Ogden, Richards: The Meaning of Meaning: A Study of the Influence of Language upon Thought and of the Science of Symbolism (1923)
„Jaguar“
Concept
apply sharedconcept
stands for
refers tosymbolizes
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How should we represent
Ontologies?
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• Classes, Relations and Instances
• Classes represent concepts
• Classes are described via attributes
• Attributes are name value pairs
Address• given name <string>• family name <string>• street <string>• ZIP code <int>• city <string>• …
„The address contains the name, title, and place of residence of the person addressed“
Informal Description
Semi informal Description
Ontologies - Components and Models
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• Relations are special attributes, whose values are objects of (other) classes
AddressPerson
Professor Student
is subclass of
Lecture
gives visits
Course
Formal Definition: Sets m1,…mn Relation R ⊆ m1 x … x mn
has an
Ontologies - Components and Models
is subclass of
is subclass of
• Classes are related to other classes
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• For Relations and Attributes Constraints (Rules) can be defined that determine allowed values
AddressPersonhas
Professor Student
1:n
1:1
Woman
Man
≠
Woman ⋂ Man = ∅Constraint
is subclass of
is subclass of
is subclass of is subclass of
Ontologies - Components and Models
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• Classes, relations, and constraints can be put together to form statements / assertions
• Special Case: formal Axioms
• Axioms describe knowledge that can‘t be expressed simply with the help of other existing components.
Example: • „it is not possible to lecture two courses at
the same time“
Ontologies - Components and Models
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• Instances describe individuals of an ontology
Course
Seminar
is subclass of
Seminar Linked DataApplication Engineering
Thursday11.00 pm A-2.1
Individuals (instances) are the basic components of an ontology. The individuals in an ontology may include concrete objects such as people, animals, tables, automobiles, molecules, and planets, as well as abstract individuals such as numbers and words.
is a
Ontologies - Components and Models
is located attakes place at
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03 How do I define a formal model of an ontology?
Open HPI - Course: Semantic Web Technologies - Lecture 4: Knowledge Representations I