Re: RDF Graphs

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

> Best
> Hugh
> -- 
> Hugh Glaser
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Received on Monday, 27 October 2014 18:06:48 UTC