Knowledge Representation Issues for the Semantic Web

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Knowledge Representation

Guest Lecture

-

CSE 335/435

Jeff Heflin

Lehigh University

Introduction


What is knowledge representation (KR)?


subfield of artificial intelligence


concerns

»
storing general purpose information

»
discovering the consequences of that information


There are many KR languages


each with its pros and cons

Semantic Networks

Penguin

Bird

Tweety

True

canFly

instanceOf

Canary

isa

False

canFly

Opus

instanceOf

isa

Frame Systems

canFly

numLegs

True

2

sings

True

isa

weight

22

instanceOf

movesBy

covering

Wings

Feathers

Bird

Canary

Tweety

slot

filler

Description Logic


Father


And(Male,Parent)


a father is a male parent


Parent


AtLeast(1, hasChild)


a parent has at least one child


Band


All(members, Musician)


every member of a band is a musician


Trio


And(All(members, Musician), Exactly(3,
members))


a trio has exactly three members, all of whom are
musicians

Description Logic Inference


subsumption


is one class a subclass of another?


calculate implicit isa relations


classification


does an object belong to a class?


example

»
bob

Male, bob

Parent

»
is bob

Father true?


Ontology


Definition


a logical theory that accounts for the intended meaning
of a formal vocabulary (Guarino 98)


has a formal syntax and unambiguous semantics


inference algorithms can compute what logically
follows


Structure


specifies at least a taxonomy


often richer definitions

Cyc Upper Ontology

From http://www.cyc.com/cycdoc/vocab/upperont
-
diagram.html

The Semantic Web


Definition


The Semantic Web is not a separate Web but an extension of the
current one, in which information is given well
-
defined meaning,
better enabling computers and people to work in cooperation.
(Berners
-
Lee et al., Scientific American, May 2001)


Applications


managing corporate web sites (intranets)


more automatic generation of web portals


better indexing of multimedia resources


web agents and web services


ubiquitous computing

A Web of Ontologies

A
1

A
2

B
3

B
1

B
2

C
1

D
1

E
1

F
1

revises

revises

revises

extends

extends

extends

extends

extends

extends

extends

S
1

S
2

S
3

S
5

S
4

commits to

commits to

commits to

commits to

commits to

commits to

RDF and RDF Schema

u:Chair

John Smith

rdf:type

g:name

<rdf:RDF


xmlns:g=“http://schema.org/gen”


xmlns:u=“http://schema.org/univ”>


<u:Chair rdf:ID=“john”>


<g:name>John Smith</g:name>


</u:Chair>

</rdf:RDF>

g:Person

g:name

rdfs:Class

rdf:Property

rdf:type

rdf:type

rdf:type

rdfs:subclassOf

rdfs:domain

<rdf:Property rdf:ID=“name”>


<rdfs:domain rdf:resource=“Person”>

</rdf:Property>



<rdfs:Class rdf:ID=“Chair”>


<rdfs:subclassOf rdf:resource=


“http://schema.org/gen#Person”>

</rdfs:Class>


URIs and Namespaces


URI


Uniform Resource Identifier


includes URLs


but also anything that you can design an identification
scheme for


helps to prevent collision of names


all the “symbols” in RDF are either URIs or Literals


Namespace


a mechanism for abbreviating URIs


by assigning a prefix for a URI fragment

OWL


Web Ontology Language


W3C Recommendation


released Feb. 2004


based on RDF

<rdf:Description rdf:about=“”>


<imports resource=“www.books.com/bookont”>

<rdf:Description>

<Book rdf:ID=“book26489”>

<author>E.B. White</author>

<title>Charlotte’s Web</title>

<price>6.99</price>

<subject rdf:resource=“&bookont;FictionChild”>

</Book>

<Class ID=“Book”>

<Property ID=“subject”>


<domain resource=“#Book”>


<range resource=“#Topic”>

</Property>

<Class ID=“FictionChild”>


<subclassOf resource=“#Fiction”>


<subclassOf resource=“#Childrens”>

</Class>



imports

bookont ontology

markup linked to semantics

semantic markup

OWL Class Constructors

borrowed from Ian Horrocks

OWL Axioms

borrowed from Ian Horrocks

OWL RDF Syntax

<owl:Class rdf:ID=”Band”>


<rdfs:subClassOf>


<owl:Restriction>


<owl:onProperty rdf:resource=”#hasMember” />


<owl:allValuesFrom resource=”#Musician” />


</owl:Restriction>


</rdfs:subClassOf>

</owl:Class>


A Band is a subset of the set of objects which only have
Musicians as members

OWL Inference

Bin Laden

<owl:Property rdf:about=“&s;head”>


<rdfs:subPropertyOf


rdfs:resource=“&s;member” />

</owl:Property>

<owl:Class rdf:about=“&t;Terrorist”>


<owl:sameClassAs>


<owl:Restriction>


<owl:onProperty


rdf:resource=“&s;member” />


<owl:someValuesFrom


rdf:resource=“&s;TerroristOrg” />


</owl:Restriction>


</owl:sameClassAs>

</owl:Class>

Al Qaeda

TerrorOrg

Terrorist

type

head

type


The head of an organization is
also a member of it


A member of a terror
organization is a terrorist


Therefore, the head of a terror
organization is a terrorist

Is the Semantic Web a Fad?


Open source Semantic Web tools


from IBM, Hewlett
-
Packard, Nokia, etc.


Commercial software vendors


Oracle 11g RDBMS supports RDF and much of OWL


Adobe’s products use RDF to provide metadata for documents, photos


Semantic Web specific companies: TopQuadrant, Aduna Software, etc.


~65 million Semantic Web documents
(a
s of October 2009)


Yahoo SearchMonkey uses RDF to present richer search results


Google now indexes RDFa (a means for embedding RDF in web pages)


Semantic Web enabled sites


Data.gov: much of U.S. government’s open data is available in RDF


Newsweek: annotates articles with RDFa


BBC Music: exports RDF playlists, RDF for all artists


Harper’s Magazine: connects articles to events on a timeline


DBPedia: a Semantic Web version of Wikipedia



Conclusion


KR has a long and impressive history


The Semantic Web is a direct descendant of
KR research


The Semantic Web is growing at an
extraordinary rate


it has potential to revolutionize the Web!

For more information...


For more on the Semantic Web


http://www.cse.lehigh.edu/~heflin/


http://www.semwebcentral.org/


http://www.w3.org/2001/sw/


http://www.semanticweb.org/


Contact me


heflin@cse.lehigh.edu

The End

Evolution of Web Standards

XML

<tr><td><b>Charlotte’s Web</b>
-

E.B. White, Garth Williams.

<font color=“Red”>$6.99</font>

</td></tr>

<book>

<title>Charlotte’s Web</title>

<author>E.B. White</author>

<author>Garth Williams</author>

<price units=“USD”>6.99</price>

<subject>Children’s Fiction</subject>

</book>

presentation
-
oriented markup

content
-
oriented markup

HTML

Species of OWL


OWL Full


very expressive (e.g., classes as instances)


theoretical properties not well understood


OWL DL


has a standard model theoretic semantics


OWL Lite


subset of OWL DL


easier to reason with

Semantic Web Challenges


The Web is distributed


many sources, varying authority


inconsistency


The Web is dynamic


representational needs may change


The Web is enormous


systems must scale well


The Web is an open
-
world