openhpi 4.2 - ontologies in computer science

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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

2

Lecture 4: Knowledge Representations I

Open HPI - Course: Semantic Web Technologies


316

02 Ontologies in Computer Science Open HPI - Course: Semantic Web Technologies - Lecture 4: Knowledge Representations I


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


516

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


616

How should we represent

Ontologies?


716

• 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


816

• 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


is subclass of

is subclass of

• Classes are related to other classes


916

• 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



1016

• 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“



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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


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

openhpi 4.2 - ontologies in computer science

Education

web technologies lecture

semanticharald sack

instances classes

nmanis subclass ofis

concepts classes

attributes attributes

basic components

existing components