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RE: The Two Dogs Problem

From: Michael Schneider <schneid@fzi.de>
Date: Sun, 19 Dec 2010 14:15:57 +0100
Message-ID: <0EF30CAA69519C4CB91D01481AEA06A002494C70@judith.fzi.de>
To: "Sampo Syreeni" <decoy@iki.fi>, "Melvin Carvalho" <melvincarvalho@gmail.com>
Cc: "Semantic Web" <semantic-web@w3.org>
Hi Melvin and Sampo!

Sampo Syreeni wrote:

>On 2010-12-17, Melvin Carvalho wrote:
>
>> Since RDF is a set, the duplicate triple is disgarded.  So Alice still
>> is described as owning one dog.
>
>Alice owns _1: which is a dog, alice owns _2: which is a dog, which are
>not necessarily the same unless by some other logic we can conflate
>them.

One has to distinguish between the syntactic and the semantic situation.

Syntactically, the graph

  G1 := {
    :alice :owns _:x1 . _:x1 rdf:type :Dog . 
    :alice :owns _:x2 . _:x2 rdf:type :Dog .
  }

consists of four pair-wise different RDF triples, i.e. it is a set of size
4. On the other hand, the graph

  G2 := {
    :alice :owns _:x1 . _:x1 rdf:type :Dog .
  }

is only a set of size 2, and therefore /syntactically/ different from G1 or,
more precisely, there is no RDF graph isomorphism modulo blank nodes between
G1 and G2.

Semantically, however, under all of the semantics of RDF(S), OWL 2 Full and
OWL 2 DL, G1 is equivalent to G2, i.e. the two graphs have exactly the same
/semantic/ meaning. The reason is that all of the mentioned semantics
interpret blank nodes (when used as anonymous individuals) as existential
variables.

Cheers,
Michael

--
Dipl.-Inform. Michael Schneider
Research Scientist, Information Process Engineering (IPE)
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Email: michael.schneider@fzi.de
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Received on Sunday, 19 December 2010 13:16:35 GMT

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