# Re: RDF graph representation

• From: Paul Gearon <gearon@ieee.org>
• Date: Fri, 23 Jul 2004 14:34:28 +1000
• Message-Id: <9570C212-DC61-11D8-81F6-000D93B0E8B6@ieee.org>
```On 23/07/2004, at 5:45 AM, Dimitri Glazkov wrote:

>
> This may be a bit of sophistry, but since predicate may be used as
> subject (and object, I assume) in RDF, doesn't this make a
> node-arc-node representation of a triple invalid?
>
> For instance, how would you represent an OWL property restriction in
> such a graph mode? Wouldn't this mean that you'd have to point an arc
> into the middle of another arc?
>
> Or am I missing something here? (which is probably the case)
>

It seems that everyone I know who is learning RDF and tries to draw a
ball and stick diagram runs into this problem (I know I did).  People
invariably seem to end up putting "balls" onto the middle of their
arcs.  :-)

A traditional directed graph has homogeneous edges, which have no
function except to connect the vertices.  These graphs are a
2-hypergraph, since each edge connects just 2 vertices.

RDF is more properly a 3-hypergraph as its edges connect 3 vertices.
To quote Wikipedia:

"A property of graphs that encourages their study, is that it is easy
to draw a representation of them on a piece of paper. This is not so
for hypergraphs and so their study tends to be conducted using the
nomenclature of set theory rather than the more pictoral descriptions
(for instance 'trees','forests' and 'cycles') of graph theory."
http://en.wikipedia.org/wiki/Hypergraph

So the problem is that we are trying to represent a 3-hypergraph as a
pseudo 2-hypergraph by labelling the edges.  As a tool it mostly works,
but gets a little messy when statements are made about predicates.  It
gets even messier with the concept of reification, as using identifiers
to represent statements starts delving into 4-hypergraphs, which get
even harder to represent pictorially.

Regards,
Paul Gearon

Software Engineer
Tucana Technologies
http://www.tucanatech.com

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immane mittam.
(Translation from latin: "I have a catapult. Give me all the money,