Re: RDF Graphs

On 27 October 2014 19:06, Pat Hayes <phayes@ihmc.us> wrote:

> The use of the word "graph" in RDF is motivated by the illustrative
> diagrams rather than by the definitions of mathematical graph theory. In
> fact, mathematical graph theory gets in the way, since RDF graphs are not
> graphs in the sense of graph theory.
>
> Formally, an RDF graph is a *set* of RDF triples, and an RDF triple is a
> 3-element *sequence* comprising a subject which is an IRI or a blank node,
> a property which is a IRI and an object which is an IRI, a blank node or a
> literal; for definitions of 'blank node' and 'literal', see
> http://www.w3.org/TR/rdf11-concepts/#section-rdf-graph
>
> So, long story short: an RDF graph is a set.
>
> On Oct 27, 2014, at 10:26 AM, Hugh Glaser <hugh@glasers.org> wrote:
>
> > Everyone talks about RDF Graphs, and I have sort of puzzled over what an
> RDF Graph is - so I thought I would ask.
> >
> > Sorry if you just need to point me at some W3C resource somewhere.
>
> The formal definitions are given in full detail in the RDF concepts
> document, part of the normative standard.
> http://www.w3.org/TR/rdf11-concepts/#section-rdf-graph.  By and large, it
> is usually good policy to read the normative standards documents to find
> the formal definitions of concepts, for any standard.
>
> >
> > "This linking structure forms a directed, labeled graph, where the edges
> represent the named link between two resources, represented by the graph
> nodes. This graph view is the easiest possible mental model for RDF and is
> often used in easy-to-understand visual explanations.”
> > (http://www.w3.org/RDF/ )
> > (I strongly agree with the second sentence, by the way!)
> >
> > Simple Graphs are usually G = (V, E) comprising a set V of vertices
> together with a set E of edges, but that doesn’t seem to describe RDF
> Graphs for me.
>
> Indeed, it does not.
>
> > The sort of thing that I am considering is an RDF Graph such as:
> >
> > rdfs:label rdfs:label “Label” .
> >
> > Is it G = ({rdfs:label, “Label”}, {(rdfs:label, “Label”)} with
> edge-labelling function (rdfs:label, “Label”) => rdfs:label ?
>
> No. It is in fact
>
>  { < www.w3.org/TR/rdf-schema/label, www.w3.org/TR/rdf-schema/label, <
> "Label", http://www.w3.org/2001/XMLSchema#string > > }
>
> > So we need to have both a vertex and an edge label with value
> rdfs:label, and they don’t really have a logical connection.
> > Sort of worrying?
> >
> > Is that the sort of graph an RDF Graph is, and is that how it is
> formally defined?
>
> No, see above.
>
> >
> > Also, a "labeled graph” usually refers to the vertices being labelled;
> should it not say that RDF is a “directed, edge-labelled graph”?
> >
> > Not exactly my forte this, so I am hoping I will be able to understand
> any answers!
>
> Hope this helps.
>

Pat is right (as usual).  You may want to add the term "directed" graph, as
I vaguely remember in graph theory edges are bidirectional by default.


>
> Pat
>
> > Best
> > Hugh
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