1. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 1 Knowledge Representation Semantic Web - Fall 2005 Computer Engineering Department Sharif University of Technology

2. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 2 2 Outline  Semantic Networks  Frames  Topic Maps  UML  RDF  First Order Logic  Conceptual Graphs  Object oriented Models  Taxonomies  Thesaurus WordNet

3. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 3 3 Explicit Specification of Knowledge  Wide variety of languages for “Explicit Specification” Graphical notations  Semantic Networks  Topic Maps  UML  RDF Logic based  Description Logics (e.g., OIL, DAML+OIL, OWL)  Rules (e.g., RuleML, LP/Prolog)  First Order Logic (e.g., KIF)  Conceptual Graphs  (Syntactically) higher order logics (e.g., LBase)  Non-classical logics (e.g., Flogic, Non-Mon, modalities) Bayesian/probabilistic/fuzzy

4. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 4 4 Semantic Networks  A semantic network is a simple representation scheme that uses a graph of labeled nodes and labeled, directed arcs to encode knowledge. Usually used to represent static, taxonomic, concept dictionaries  Semantic networks are typically used with a special set of accessing procedures that perform “reasoning” e.g., inheritance of values and relationships  Semantic networks were very popular in the ‘60s and ‘70s but are less frequently used today. Often much less expressive than other KR formalisms  The graphical depiction associated with a semantic network is a significant reason for their popularity.

5. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 5 5 A Semantic Network Example Joe Boy Kay Woman Food Human Being School Has a child Needs Goes to Is a

6. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 6 6 From Semantic Nets to Frames  Semantic networks morphed into Frame Representation Languages in the ‘70s and ‘80s.  A frame is a lot like the notion of an object in OOP, but has more meta-data.  A frame has a set of slots.  A slot represents a relation to another frame (or value).  A slot has one or more facets.  A facet represents some aspect of the relation.

7. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 7 7 Facets  A slot in a frame holds more than a value.  Other thing facets might include: default fillers minimum and maximum number of fillers type restriction on fillers (usually expressed as another frame object) attached procedures (if-needed, if-added, if- removed) attached constraints or axioms …  In some systems, the slots themselves are instances of frames.

8. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 8 8

9. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 9 9 Topic Maps  A set of linked topics that index a document collection.  Optimized for navigation  ISO standard (ISO 13250)

10. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 10 10 Topic Maps (contd.)  Topic Maps is a technology that has arisen in recent years to address the issue of semantically characterizing and categorizing documents and sections of documents on the Web with respect to their content—in other words, what topics or subject areas those documents actually address.  Concepts: Topic  any distinct subject of interest for which assertions can be made. Occurrence  a resource specifying some information about a topic. Association  the relationship between (one or more) topics. Scope  Similar to namespace in other markup languages.

11. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 11 11 UML

12. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 12 12 RDF

13. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 13 13 First Order Logic

14. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 14 14 Conceptual Graphs

15. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 15 15 Many languages use “object oriented” model based on:  Objects/Instances/Individuals Elements of the domain of discourse Equivalent to constants in FOL  Types/Classes/Concepts Sets of objects sharing certain characteristics Equivalent to unary predicates in FOL  Relations/Properties/Roles Sets of pairs (tuples) of objects Equivalent to binary predicates in FOL  Such languages are/can be: Well understood Formally specified (Relatively) easy to use Amenable to machine processing Object Oriented Models

16. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 16 16 Taxonomies  The classification of information entities in the form of a hierarchy, according to the presumed relationships of the real-world entities that they represent.

17. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 17 17 UNSPSC  United Nations Standard Products and Services Classification  A well-known taxonomy used in electronic commerce.  A Portion of the UNSPSC Electronic Commerce Taxonomy:

18. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 18 18 Why use taxonomies?  Browsing and Navigating information, especially when you only have a general idea of what you are looking for.  A way of structuring your data and your information entities. On the Web, taxonomies can be used to help your customers find your products and services.  A directory or registry of Web products and services absolutely needs some way of classifying those products and services; otherwise, how can anything be found?

19. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 19 19 Thesaurus  A controlled vocabulary arranged in a known order and structured so that equivalence, homographic, hierarchical, and associative relationships among terms are displayed clearly and identified by standardized relationship indicators.  The primary purposes of a thesaurus are to facilitate retrieval of documents and to achieve consistency in the indexing of written or otherwise recorded documents and other items.  A thesaurus is typically used to associate the rough meaning of a term to the rough meaning of another term.

20. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 20 20 Semantic Relations of a Thesaurus

21. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 21 21 WordNet  http://wordnet.princeton.edu/ http://wordnet.princeton.edu/  A word in WordNet thesaurus has the information typically associated with it : Synonyms—Those nodes in the taxonomy that are in the “mean the same as” relation. Hypernyms—Those nodes that are in the “parent of” or “broader than” relation in the taxonomy; if the taxonomy is a tree, there is only one parent node. Hyponyms—Those nodes that are in the “children of” or “narrower than” relation;

22. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 22 22

23. Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 23 23 References  Chapter 7 of the book  http://wordnet.princeton.edu/ http://wordnet.princeton.edu/