University Politehnica of Bucharest

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Artificial Intelligence

• University Politehnica of Bucharest
• 2008-2009
• Adina Magda Florea
• http//turing.cs.pub.ro/aifils_08

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Structured knowledge representation
Lecture No. 8

• Semantic Networks
• Units
• Specific inferences
• Problems with inheritance
• Semantic Web

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Semantic networks

• first model for structured knowledge
  representation
• describe semantics of natural language
• used extensively as a model for representing
  knowledge in KBS

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Knowledge base

• Radu sent a letter to Maria.
• Radu is a student.
• Ioana is a schoolgirl.
• Radu's address is Luterana, 15.

• occupation (radu, student)
• occupation (ioana, schoolgirl)
• send (radu, ioana, letter)
• address (radu, luterana - 15)

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Group knowledge into entities

• Radu
• occupation (radu, student)
• send (radu, ioana, letter)
• address (radu, luterana - 15)
• Ioana
• occupation (ioana, schoolgirl)
• send (radu, ioana, letter)

Associate properties or attributes
Model 2 args predicates
Radu occupation student address
luterana-15 Ioana occupation schoolgirl
What about 3 args predicates?
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- use ISA predicate - use Skolemization to
eliminate existential quantifier
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Radu ISA Person Occupation student
Address luterana-15 Ioana ISA Person
Occupation schoolgirlT1 ISA Sending-event
Sender Radu Receiver Ioana Object letter
slots
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ISA introduces a class-instance relationship AKO
predicate introduces a subclass-class
relationship
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Sending-event AKO Event Sender Person Rec
eiver Person Object ObjectClassPerson AK
O living Thing Occupation (student, engineer,
) Address string
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Examples of SN
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Units

• Units
• MemberOf - ISA
• SubClass, SuperClass - AKO

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Facets

• Facets
• Features associated to slots
• Possible facets
• Value facet
• Value type facet
• Default facet
• If-needed procedure or function
• Demon facet
• Comment facet

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Specific inferences in semantic networks
properties/attributes inheritance 1)
Inheritance of attributes along the ISA relation
2) Inheritance of attributes along the AKO
relation Attribute's value inheritance
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Attribute's value inheritance
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Algorithm Inheritance of attribute's value in a
class hierarchy The algorithm gets the value V
of an attribute A of the object OFindVal (O,
A, V)1. Create a list L with node O and all the
nodes linked to O by an ISA relationship2. while
L ! do 2.1. Remove first node N from L
2.2. if attribute A of node N has value, be it
V then 2.2.1. Place V as the value of
attribute A of object O 2.2.3. return
SUCCESS 2.3. Add all nodes linked by AKO to N at
the end of L3. return FAILend.
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Attribute's default value inheritance
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Algorithm Inheritance of attribute's default
value in a class hierarchy The algorithm gets
the default value V of an attribute A of the
object OFindDefault (O, A, V)1. Create a list L
with node O and all the nodes linked to O by an
ISA relationship2. while L !
do 2.1. Remove first node N from L 2.2. if
attribute A of node N has a default value, be it
V then 2.2.1. Place V as the value of
attribute A of object O 2.2.3. return
SUCCESS 2.3. Add all nodes linked by AKO to N at
the end of L3. return FAILend.
FindIfNeeded(O,A,V)
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Control strategy

• the control strategy of inheritance indicates the
  order of considering the different facets
• 2 basic strategies
• N strategy
• Z strategy

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Algorithm The algorithm gets the value V of an
attribute A of the object O using the N
strategyFindValN (O, A, V)1. if FindVal
(O,A,V) SUCCESS then return SUCCESS2. if
FindDefault (O,A,V) SUCCESS then return
SUCCESS3. if FindIfNeeded (O,A,V) SUCCESS
then return SUCCESS4. return FAILend
Strategy N
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Algoritm Strategia Z de determinare a valorii
unui atribut. Algoritmul determina valoarea
unui atribut A al unei instante O utilizind
strategia Z.DetValZ (O, A, V)1. Formeaza o
lista L cu nodul O si toate nodurile legate de
O prin relatia ISA2. cat timp L !
executa 2.1. Elimina primul nod, N, din lista
L 2.2. daca fateta valoare a atributului A a
nodului N este V then 2.2.1. Depune V
in nodul punctat de atributul A al
obiectului O 2.2.2. intoarce SUCCES
Strategy Z
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2.3. daca fateta valoare implicita a atributului
A a nodului N este V atunci 2.3.1. Depune
V in nodul punctat de atributul A al
obiectului O 2.3.2. intoarce
SUCCES 2.4. daca fateta procedura necesara a
atributului A a nodului N este proc (A1,...,
An,V) atunci 2.4.1. Determina valorile
atributelor A1,..., An ale instantei
O 2.4.2. daca s-au gasit valori pentru A1,...,
An atunci i. executa proc (A1,...,
An,V) ii. Depune V in nodul punctat de
atributul A al obiectului O iii. intoarce
SUCCES3. intoarce INSUCCESsfarsit.
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Problems with inheritance

• DAG networks
• Multiple inheritance

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(No Transcript)
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Multiple inheritance

• Distance between units
• If we look for the value of an attribute A of a
  unit U1 and find 2 paths U1?U2 and U1?U3 to U2
  and U3 containing values for A
• then compute the length of the alternate paths
  from the instance/unit U1 to units U2 and U3 and
  choose the value from the closest unit (shortest
  path)
• May cause semantic problems

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Shortest distance
Shortest inferential distance
Distance and inferential distance
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Multiple inheritance

• Inferential distance
• Unit1 is closer to Unit2 than to Unit3 if and
  only if Unit1 has an inferential path to Unit3
  which contains Unit2.
• Unit1 is closer to Unit2 than to Unit3 if and
  only if Unit2 is on an inferential path of ISA
  and AKO relationships between Unit1 and Unit3

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Algorithm Inheritance of attribute's value in a
class hierarchy based on inferential
distanceFind the value V of slot S of unit
U1. Make a list L with unit U and all units to
which unit U is linked by an ISA/MemberOf2. Init
ialize a list CAND 3. while L !
do 3.1. remove first unit X from L 3.2. if slot
S of X has value then CAND CAND
?X 3.3. else add to L all units to which X is
linked by a AKO/SuperClass4. for each unit C ?
CAND do 4.1. Verify if there is another C ?
CAND having an inferential distance to U
shorter than to C 4.2. if such a C'
exists then remove C from CAND
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5. if card (CAND) 0 then return FAIL / no
value for S /6. if card (CAND) 1 then
6.1. Be C the one element of CAND 6.2. Make
the value V of slot S of C the value of slot S
of U 6.3. return SUCCESS7. if card (CAND) gt 1
then return CONTRADICTIONend.
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Combined representationsKnowledge base
contains - declarative knowledge units -
procedural knowledge rules daca
Camion.Inaltime gt 2 si Camion.Culoare
rosu atunci Camion.Pret 1000
Inferences both specific to units and specific
to rules
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Semantic Web and Ontologies
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Semantic Web

• Web was invented by Tim Berners-Lee (amongst
  others), a physicist working at CERN
• TBLs original vision of the Web was much more
  ambitious than the reality of the existing
  (syntactic) Web
• TBL (and others) have since been working towards
  realising this vision, which has become known as
  the Semantic Web
• E.g., article in May 2001 issue of Scientific
  American

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Where we are Today the Syntactic Web
Hendler Miller 02
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The Syntactic Web is

• A hypermedia, a digital library
• A library of documents called (web pages)
  interconnected by a hypermedia of links
• A database, an application platform
• A common portal to applications accessible
  through web pages, and presenting their results
  as web pages
• A platform for multimedia
• BBC Radio 4 anywhere in the world! Terminator 3
  trailers!
• A naming scheme
• Unique identity for those documents
• A place where computers do the presentation
  (easy) and people do the linking and interpreting
  (hard).
• Why not get computers to do more of the hard
  work?

Goble 03
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Impossible (?) using the Syntactic Web

• Complex queries involving background knowledge
• Find information about animals that use sonar
  but are not either bats or dolphins
• Locating information in data repositories
• Travel enquiries
• Prices of goods and services
• Results of human genome experiments
• Delegating complex tasks to web agents
• Book me a holiday next weekend somewhere warm,
  not too far away, and where they speak French or
  English

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What is the Problem?

• Consider a typical web page

• Markup consists of
• rendering information (e.g., font size and
  colour)
• Hyper-links to related content
• Semantic content is accessible to humans but not
  (easily) to computers

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What information can we see

• WWW2002
• The eleventh international world wide web
  conference
• Sheraton waikiki hotel
• Honolulu, hawaii, USA
• 7-11 may 2002
• 1 location 5 days learn interact
• Registered participants coming from
• australia, canada, chile denmark, france,
  germany, ghana, hong kong, india, ireland, italy,
  japan, malta, new zealand, the netherlands,
  norway, singapore, switzerland, the united
  kingdom, the united states, vietnam, zaire
• Register now
• On the 7th May Honolulu will provide the backdrop
  of the eleventh international world wide web
  conference. This prestigious event
• Speakers confirmed
• Tim berners-lee
• Tim is the well known inventor of the Web,
• Ian Foster
• Ian is the pioneer of the Grid, the next
  generation internet

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What information can a machine see

• WWW2002
• The eleventh international world wide web
  conference
• Sheraton waikiki hotel
• Honolulu, hawaii, USA
• 7-11 may 2002
• 1 location 5 days learn interact
• Registered participants coming from
• australia, canada, chile denmark, france,
  germany, ghana, hong kong, india, ireland, italy,
  japan, malta, new zealand, the netherlands,
  norway, singapore, switzerland, the united
  kingdom, the united states, vietnam, zaire
• Register now
• On the 7th May Honolulu will provide the backdrop
  of the eleventh international world wide web
  conference. This prestigious event
• Speakers confirmed
• Tim berners-lee
• Tim is the well known inventor of the Web,
• Ian Foster
• Ian is the pioneer of the Grid, the next
  generation internet

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Solution XML markup with meaningful tags?

• ltnamegtWWW2002
• The eleventh international world wide
  webconlt/namegt
• ltlocationgtSheraton waikiki hotel
• Honolulu, hawaii, USAlt/locationgt
• ltdategt7-11 may 2002lt/dategt
• ltslogangt1 location 5 days learn interactlt/slogangt
• ltparticipantsgtRegistered participants coming from
• australia, canada, chile denmark, france,
  germany, ghana, hong kong, india, ireland, italy,
  japan, malta, new zealand, the netherlands,
  norway, singapore, switzerland, the united
  kingdom, the united states, vietnam,
  zairelt/participantsgt
• ltintroductiongtRegister now
• On the 7th May Honolulu will provide the backdrop
  of the eleventh international world wide web
  conference. This prestigious event
• Speakers confirmedlt/introductiongt
• ltspeakergtTim berners-leelt/speakergt
• ltbiogtTim is the well known inventor of the
  Web,lt/biogt

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But What About

• ltconfgtWWW2002
• The eleventh international world wide
  webconlt/confgt
• ltplacegtSheraton waikiki hotel
• Honolulu, hawaii, USAlt/placegt
• ltdategt7-11 may 2002lt/dategt
• ltslogangt1 location 5 days learn interactlt/slogangt
• ltparticipantsgtRegistered participants coming from
• australia, canada, chile denmark, france,
  germany, ghana, hong kong, india, ireland, italy,
  japan, malta, new zealand, the netherlands,
  norway, singapore, switzerland, the united
  kingdom, the united states, vietnam,
  zairelt/participantsgt
• ltintroductiongtRegister now
• On the 7th May Honolulu will provide the backdrop
  of the eleventh international world wide web
  conference. This prestigious event
• Speakers confirmedlt/introductiongt
• ltspeakergtTim berners-leelt/speakergt
• ltbiogtTim is the well known inventor of the Web,

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Must add semantics

• External agreement on meaning of annotations
• E.g., Dublin Core
• Agree on the meaning of a set of annotation tags
• Problems with this approach
• Inflexible
• Limited number of things can be expressed
• Use Ontologies to specify meaning of annotations
• Ontologies provide a vocabulary of terms
• New terms can be formed by combining existing
  ones
• Meaning (semantics) of such terms is formally
  specified
• Can also specify relationships between terms in
  multiple ontologies

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Ontology in philosophy

• a philosophical disciplinea branch of philosophy
  that
• deals with the nature and the organisation of
  reality
• Science of Being (Aristotle, Metaphysics, IV, 1)
• Tries to answer the questions
• What characterizes being?
• Eventually, what is being?

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Ontology in Computer Science

• An ontology is an engineering artifact
• It is constituted by a specific vocabulary used
  to describe a certain reality, plus
• a set of explicit assumptions regarding the
  intended meaning of the vocabulary.
• Thus, an ontology describes a formal
  specification of a certain domain
• Shared understanding of a domain of interest
• Formal and machine manipulable model of a domain
  of interest
• An explicit specification of a
  conceptualisation Gruber93

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Structure of an ontology

• Ontologies typically have two distinct
  components
• Names for important concepts in the domain
• Elephant is a concept whose members are a kind of
  animal
• Herbivore is a concept whose members are exactly
  those animals who eat only plants or parts of
  plants
• Adult_Elephant is a concept whose members are
  exactly those elephants whose age is greater than
  20 years
• Background knowledge/constraints on the domain
• Adult_Elephants weigh at least 2,000 kg
• All Elephants are either African_Elephants or
  Indian_Elephants
• No individual can be both a Herbivore and a
  Carnivore

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Example of ontology
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Tools

• Description Logics
• OWL Web Ontological Language
• Protégé framework for building ontologies