W3C home > Mailing lists > Public > www-rdf-logic@w3.org > September 2001

Re: more on labeled graphs

From: Wolfgang Nejdl <nejdl@kbs.uni-hannover.de>
Date: Fri, 28 Sep 2001 23:50:50 +0200
Message-Id: <200109282150.f8SLooO05638@liszt.kbs.uni-hannover.de>
To: Pat Hayes <phayes@ai.uwf.edu>
cc: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>, www-rdf-logic@w3.org
> >From: Pat Hayes <phayes@ai.uwf.edu>
> >Subject: Re: model theory for RDF/S
> >Date: Thu, 27 Sep 2001 19:42:28 -0500
> >
> >>  But why bother with all this? The concept of a labelled graph is
> >>  standard and uncontroversial, so this is one place where we can say
> >>  some mathematics in a reasonably intuitive way without sacrificing
> >>  either precision or readability by non-mathematicians.
> >
> >I am aware of at least four versions of labeled directed graphs.  (This
> >ignores hypergraphs.)  These different versions of labeled graphs differ on
> >how they treat edges, and have different implications for the RDF model
> >theory. ...
> 
> OK, I agree that this needs to be clarified. (I definitely think that 
> we should avoid multigraphs. The subgraph lemma could be adapted to 
> this case, but it creates needless complications, and makes it very 
> hard to draw pictures.)
> 
> Ive checked several web sources of references, by the way, and found, 
> amusingly enough, that they often *describe* graphs in a way that 
> suggests the many-edges interpretation, but give formal results and 
> definitions that assume the single-edge one (eg assertions that there 
> are at most n|2 graphs on a set of n vertices), which suggests that 
> this misunderstanding is quite widespread.
> 
> The possibility of unattached nodes in an RDF graph is quite 
> interesting.  We could take the position that such a node is a simple 
> assertion of existence, so that each node contains a kind of implicit 
> assertion of self-identity. This has a nice echo both of CS Peirce 
> and of the rather curious usage in some W3C documents where a 
> resource is defined as anything that has an "identity".
> 

Just to compare with related formalisms: O-Telos (and O-Telos-RDF,
http://www.kbs.uni-hannover.de/Arbeiten/Publikationen/2001/kcap01_final.pdf)
allows these unattached nodes, and treats them as assertions of
existence. On the other points mentioned in the preceding discussion:
O-Telos/O-Telos-RDF does not make any distinction between literals and
other types, and labeled graphs in O-Telos/O-Telos-RDF are single-edge
ones (but this is more natural (in contrast to RDF(S)), because
instantiated properties (edges) get a new label instead of the name of
the property).

Wolfgang

> But it would be simpler just to make them illegal.
> 
> Pat
> -- 
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Prof. Dr. Wolfgang Nejdl              tel. +49 511 762-19710
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Received on Friday, 28 September 2001 17:53:32 GMT

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