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

Re: more on labeled graphs

From: Pat Hayes <phayes@ai.uwf.edu>
Date: Fri, 28 Sep 2001 14:54:10 -0500
Message-Id: <p0510102fb7da79ffce22@[205.160.76.184]>
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>
Cc: 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".

But it would be simpler just to make them illegal.

Pat
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Received on Friday, 28 September 2001 15:54:20 UTC

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