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Re: RDF Graphs

From: Peter F. Patel-Schneider <pfpschneider@gmail.com>
Date: Mon, 27 Oct 2014 09:39:08 -0700
Message-ID: <544E752C.3050903@gmail.com>
To: Hugh Glaser <hugh@glasers.org>, Antoine Zimmermann <antoine.zimmermann@emse.fr>
CC: semantic-web@w3.org
There are many notions of graphs.

An RDF graph is not a simple graph, i.e., a set of pairs.

However, an RDF graph is a a kind of graph, namely a multi-graph where the 
nodes have optional unique labels and the edges have labels and there is at 
most one edge with a given label between two nodes.  RDF graphs also have a 
special kind of node that cannot be the source of an edge.  This all 
translates into the official view of RDF graphs in the W3C recommendation.

peter


On 10/27/2014 09:28 AM, Hugh Glaser wrote:
> Thanks Antoine.
>
> So "an RDF Graph is not a graph (in the usual mathematical sense)”.
> Do we all agree on that?
>
> But if it is, it is a "directed (edge-and-node-)labelled multigraph”, which is a much better description than "directed, labeled graph” from http://www.w3.org/RDF/
>
> Cheers
>
>> On 27 Oct 2014, at 15:44, Antoine Zimmermann <antoine.zimmermann@emse.fr> wrote:
>>
>> The abstract of RDF 1.1 Concepts and Abstract Syntax says:
>>
>> "RDF graphs are sets of subject-predicate-object triples, where the elements may be IRIs, blank nodes, or datatyped literals."
>>
>> http://www.w3.org/TR/rdf11-concepts/#abstract
>>
>> The more formal definition is given in Section 3:
>>
>> "An RDF graph is a set of RDF triples."
>>
>> and:
>>
>> """
>> An RDF triple consists of three components:
>> - the subject, which is an IRI or a blank node
>> - the predicate, which is an IRI
>> - the object, which is an IRI, a literal or a blank node
>> """
>>
>> http://www.w3.org/TR/rdf11-concepts/#dfn-rdf-graph
>>
>>
>> So, in fact, an RDF Graph is not a graph (in the usual mathematical sense), but it can be mapped to a directed (edge-and-node-)labelled multigraph.
>>
>> In your exemple, G is the singleton {(rdfs:label,rdfs:label,"Label")}.
>>
>>
>> --AZ.
>>
>>
>>
>> Le 27/10/2014 16:26, Hugh Glaser a écrit :
>>> 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.
>>>
>>> "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.
>>>
>>> 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 ?
>>> 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?
>>>
>>> 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!
>>> Best
>>> Hugh
>>>
>>
>> --
>> Antoine Zimmermann
>> ISCOD - Institut Henri Fayol
>> École des Mines de Saint-Étienne
>> 158 cours Fauriel
>> CS 62362
>> 42023 Saint-Étienne Cedex 2
>> France
>> Tél:+33(0)4 77 42 66 03
>> Fax:+33(0)4 77 42 66 66
>> http://zimmer.aprilfoolsreview.com/
>>
>
Received on Monday, 27 October 2014 16:39:42 UTC

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