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RE: stating that something doesn't exist

From: Johnson, Matthew C. (LNG-HBE) <Matthew.C.Johnson@lexisnexis.com>
Date: Wed, 21 May 2008 22:18:29 -0400
Message-ID: <0FE5E87C5F0AE84B8C667FDC5224F6DA0229F081@LNGDAYEXCP01VC.legal.regn.net>
To: "Peter Ansell" <ansell.peter@gmail.com>
Cc: <semantic-web@w3.org>

Hmm.  This is interesting, thanks.  As far as generating a lot of
triples from a sparsely linking set of publications, I think my actual
use-case would probably eliminate that.  I suspect that this type of
query would normally be done on a publication-by-publication basis.  So
I would really be checking if any links exist specifically for
publication 2 to [specifically] publication 3.  This would dramatically
reduce the number of triples generated (although it might still take
some time).

Also, I suspect another alternative would be to let the application
itself do some of the rationalization by simply grabbing all of the
publications that "pub 2" does link to (a simple query) and then
checking to see if "pub 3" is in that list.  Ideally, I would like the
data or query to say this directly (really, I'd just like a yes or no
answer) but if performance becomes an issue, it might be a second

Thanks again for the thoughts.


-----Original Message-----
From: Peter Ansell [mailto:ansell.peter@gmail.com] 
Sent: Wednesday, May 21, 2008 5:30 PM
To: Johnson, Matthew C. (LNG-HBE)
Cc: semantic-web@w3.org
Subject: Re: stating that something doesn't exist

2008/5/22 Johnson, Matthew C. (LNG-HBE)
> Hi,
> I'm slowing soaking in what OWL can and cannot do and after reading
> "Lessons For Ontology Writers" post [1] by Ian Davis, I thought I'd go
> and ask this capabilities question here.
> In OWL, can you directly define a property that says that something
does not
> occur.  My current use-case is in determining hyper-linking
> between various publications.  The following:
> mysch:Pub a rdfs:Class .
> mysch:linksTo a rdf:Property .
> mypubs:p1 a mysch:Pub .
> mypubs:p2 a mysch:Pub .
> mypubs:p3 a mysch:Pub .
> mypubs:p1 mysch:linksTo mypubs:2 .
> mypubs:p3 mysch:linksTo mypubs:2 .
> Lets me say that pub 1 links to pub 2 and that pub 3 links to pub 2.
> However, if I understand the open-world assumption, I cannot assume
that pub
> 2 does not link to pub 3.  I would need to define something like:
> mysch:notLinksTo a rdf:Property .
> to explicitly state this fact that pub 2 does not link to pub 3.
> My question is whether it is possible in OWL to define a property
> range of acceptable instances is the list of instances that do not
exist as
> an object in a mysch:linksTo statement (for a given publication)?
> statically stating that pub 2 does not link to pub 3 is technically
> for me, it seems unfortunate.  This is because when a new pub comes
> (e.g. mysch:p4), I then have to update all the statements I've already
> made.  This is the part that Ian Davis agreed would cause an explosion
> triples.  I would really like to only state the things that I can
> programmatically find by parsing the raw syntax of a publication (e.g.
I can
> find a <a href...> element which provides the mysch:linksTo assertion)
>  would rather the system infer that the lack of a <mysch:linksTo>
> relationship really means a <mysch:notLinksTo>.
> Perhaps the answer is really through SPARQL.  I hope I've made my
> question(s) clear.  Please let me know if I haven't.

You can use SPARQL to partially derive information about the open
world by using the following construct:

SELECT DISTINCT ?unlinkedDocument1, ?unlinkedDocument2
# pick out two publications
?unlinkedDocument1 a mysch:Pub
?unlinkedDocument2 a mysch:Pub

# make them unique
FILTER(?unlinkedDocument1 != ?unlinkedDocument2)

# Introduce a temporary variable in an optional section to determine
whether the pattern exists for either document
OPTIONAL{ ?temporaryDocument1 mysch:linksTo ?unlinkedDocument2
FILTER(?temporaryDocument1 = ?unlinkedDocument1))

OPTIONAL{ ?temporaryDocument2 mysch:linksTo ?unlinkedDocument1
FILTER(?temporaryDocument2 = ?unlinkedDocument2))

# Check off that both of the temporary variables from the OPTIONAL
parts didn't bind, meaning there was no link, and hence including
these two documents in the results as a row where the two fields were
not linked
FILTER(!bound(?temporaryDocument1) && !bound(?temporaryDocument2})

That should do what you want. And I know, it looks like magic but it
seems to work. It would create a rather large set of results on any
sufficiently large and sparsely linked set of publications, so running
it may take some time and generate a lot of results!

Hope that helps,

Received on Thursday, 22 May 2008 02:19:21 GMT

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