Re: [ontolog-forum] Interpreting OWL

[Pavithra <pavithra_kenjige@xxxxxxxxx>]

Dr. Sowa and  Rich,
In professor and student situation,  a directed graph is established taking  the period of eduction or course of study into consideration,  with an established formal relationship about who teaches the course, and who is enrolled to study as a students for that course.   So it is not about what each person is,  (does not matter if the student  has been professor before or teaching a class somewhere else.  It is pertinent to course of study and duration of the study.    Because without the course of study,  a professor and student relationship does not exist.   But self study study situation establishes a student with a course of study with a direct relationship with the professor.   These techniques are used to avoid the loop or cyclic situation..    But these are simple examples compare to chemical compound,

Pavithra

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From: John F. Sowa <sowa@xxxxxxxxxxx>
To: ontolog-forum@xxxxxxxxxxxxxxxx
Sent: Sun, October 17, 2010 8:41:06 PM
Subject: Re: [ontolog-forum] Interpreting OWL

On 10/17/2010 5:29 PM, Pavithra wrote:
> What is the need to support acyclic structure vs cyclic ?

Any collection of relations among a discrete set of entities
can be represented by a labeled directed graph -- or by a
collection of tables, as in an RDB.  (If there are infinitely
many entities, the graphs and tables may have to be infinite.)

For some applications, those graphs (or tables) don't have
cycles.  But most applications (even for something as simple
as a benzene molecule) require graphs that have cycles.
To guarantee decidability of their inferencing methods, OWL
imposes constraints that make it impossible to say or imply
that the structure has cycles.

For some applications, it's possible to live with that restriction.
But for most applications, it's necessary to have a more general
knowledge representation language than OWL.

On 10/17/2010 6:21 PM, Rich Cooper wrote:
> But wouldn’t that hierarchy [of professors & grad students]
> be a lattice instead of a tree?

That depends on what relation between prof & students you intend
to represent.  The relation (X took a course from Y) could have
cycles.  But the relation (X earned a PhD with Y as thesis advisor)
would be a tree, except in rather unusual circumstances.

John





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