Re: [RDF-Concepts:109] What is the expressive power of RDF?

From: Graham Klyne <GK@ninebynine.org>
Date: Fri, 21 Feb 2003 10:57:23 +0000
Message-Id: <5.1.0.14.2.20030221105011.00aabec0@127.0.0.1>
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>, Brian McBride <bwm@hplb.hpl.hp.com>

Peter,

Thank you for your clarification.  I apologize for missing the second half

I think there are two issues here:

1. Greater care is needed in discussing the expressive power of RDF.

Brian, can you please raise an issue for this.

2. How can the RDF and RDFS semantic conditions be represented in the
existential-conjunctive subset of first order logic?

I am not aware that this was ever claimed; indeed, I don't think it is
possible to do this.  So I do not understand why this is a concern.

#g
--

At 01:43 PM 2/19/03 -0500, Peter F. Patel-Schneider wrote:

>From: Graham Klyne <GK@ninebynine.org>
>Subject: [RDF-Concepts:109] What is the expressive power of RDF?
>Date: Wed, 19 Feb 2003 16:05:36 +0000
>
> > Peter,
> >
> > With reference to your comment:
> >
> >
> > I accept a need for editorial revision, and have recorded it with id
> > 109-ExpressivePower [**].  The purpose of the rest of this message is to
> > try to ensure that I properly understand your concerns.
> >
> > [**] for my own tracking purposes:  when the scope of the issue is
> > established, I'll ask Brian to allocate a WG tracking ID.
> >
> > I think the problem can be described thus:
> > [[
> > There is incorrect wording describing the expressive power of RDF.
> >
> > A formal description would be:
> > "The expressive power of RDF is equivalent to the binary
> > existential-conjunctive subset of first order logic".
> >
> > Any informal explanations should be consistent with this.
> > ]]
> >
> >
> > To further help me understand your concerns, can you clarify to me why you
> > regard the following representations are not legitimate answers to the
> > questions you raise:
> >
> > [[
> > 1. How can
> >     takes(John,book,school)
> >     be represented in RDF?
> >
> >     <rdf:Description>
> >         <rdf:type rdf:resource="ex:TakingEvent" />
> >         <ex:taker rdf:resource="ex:John"/>
> >         <ex:taken rdf:resource="ex:Book"/>
> >         <ex:to    rdf:resource="ex:School"/>
> >     </rdf:Description>
>
>This is an encoding of a trinary relationship as several binary
>relationships.  If you wish to admit such encodings, then I think that you
>should be much more formal about ``expressive power''.  It may be that
>under some suitable definition of ``expressive power'' RDF can express
>n-ary relationships.  However, under other definitions of ``expressive
>power'' the above encoding is not admissable.  For example, the above
>encoding allows for TakingEvents that do not have a taker, but the trinary
>takes predicate does not admit this possibility.
>
> > 2. How can
> >     loves(John,spouse(John))
> >     be represented in RDF?
> >
> >     <rdf:Description about="ex:John">
> >        <ex:loves rdf:parseType="resource">
> >           <rdf:Description>
> >              <ex:spouse rdf:resource="ex:John" />
> >           </rdf:Description>
> >        </ex:loves>
> >     </rdf:Description>
> > ]]
>
>This is not even an encoding, as it is missing the functionality of spouse.
>
>
>You haven't addressed the second half of this comment.
>
> > #g
>
>Peter F. Patel-Schneider
>Bell Labs Research
>Lucent Technologies
>
>
>
> > At 10:26 AM 1/30/03 -0500, Peter F. Patel-Schneider wrote:
> >
> >
> > >RDF Concepts states
> > >         The expressive power of RDF corresponds to the
> > >         existential-conjunctive (EC) subset of first order logic [Sowa].
> > >
> > >How can
> > >         takes(John,book,school)
> > >be represented in RDF?
> > >
> > >How can
> > >         loves(John,spouse(John))
> > >be represented in RDF?
> > >
> > >How can the RDF and RDFS semantic conditions be represented in the
> > >existential-conjunctive subset of first order logic?
> >
> > -------------------
> > Graham Klyne
> > <GK@NineByNine.org>

-------------------
Graham Klyne
<GK@NineByNine.org>
Received on Friday, 21 February 2003 13:07:24 UTC

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