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RDF Semantics, non-lean RDF graphs, and redundancy of content

From: Ossi Nykänen <onykane@butler.cc.tut.fi>
Date: Thu, 4 Dec 2003 11:12:41 +0200 (EET)
To: www-rdf-comments@w3.org
Message-ID: <Pine.GSO.4.58.0312041103360.180@butler.cc.tut.fi>

Dear all,

I have a minor comment to the characterisation of non-lean RDF graphs. In
short, I wonder what "redundancy" means in the context of non-lean RDF
graphs (in RDF Semantics).

The longer version:

In ...

RDF Semantics
W3C Working Draft 10 October 2003
http://www.w3.org/TR/2003/WD-rdf-mt-20031010/#graphdefs

...the text reads (graph labels and blocks added):

----------snip-
An RDF graph is lean if it has no instance which is a proper subgraph of
the graph. Non-lean graphs have internal redundancy and express the same
content as their lean subgraphs.
For example, the graph

G1: {
<ex:a> <ex:p> _:x .
_:y <ex:p> _:x . }

is not lean, but

G2: {
<ex:a> <ex:p> _:x .
_:x <ex:p> _:x . }

is lean.
----------snip-

I don't quite understand the sentence "Non-lean graphs have internal
redundancy and express the same content as their lean subgraphs." From the
modelling point of view, this seems rather important. Perhaps the concept
"redundancy" should be defined in this context?

What kind of example would illustrate the "removal or redundancy"? G2 is
an instance of G1 but G2 is not a subgraph of G1 (two different blank
nodes would be accidentally identified) so the example in the text will
not do(?)

I'm asking this because I can't figure out how to write the content of G1
"without redundancy" -- all changes seem to change the design and thus the
potential interpretations (e.g. G1 has more models than G2). (The
definition of an instance does not assume the vocabulary of the particular
graph. This seems sensible since an agent might have a rich inner
vocabulary which it tries to match with the blank nodes?)

Perhaps the text should read something like: "If a non-lean graph A has a
lean subgraph B, then A and B express the same content i.e. A is
redundant." ???
(And personally, I would still be interested to see an example how to
reduce "redundancy" from G1 without affecting the content.)

What am I missing? (Expect for common sense, of course.) I can only
suspect that I "disagree" either about the concept "redundancy" or about
the role of blank nodes in taking subgraphs.

Best regards,

--Ossi


--
Ossi Nykänen                              Tel   +358 3 3115 3544
Tampere University of Technology          Fax   +358 3 3115 3549
DMI / W3C Finnish Office                  Email ossi@w3.org
P.O. Box 553, FIN-33101 Tampere, Finland  Web   http://www.w3c.tut.fi
Received on Thursday, 4 December 2003 04:12:49 GMT

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