- From: <herman.ter.horst@philips.com>
- Date: Fri, 7 Nov 2003 13:42:16 +0100
- To: www-rdf-comments@w3.org

I would like to make two comments about the RDF Concepts document. "In the RDF abstract syntax, a blank node is just a unique node that can be used in one or more RDF statements, and has no globally distinguishing identity." It seems that the use of the word "globally" could use a little further explanation. A blank node has a unique identity within an RDF graph (or within an RDF document), which can be a very large thing. There is some analogy with a global variable in a computer program. However, a blank node is not distinguishing over different RDF graphs. === It seems that the definition of graph equivalence could undergo some editorial improvement, by mentioning explicitly that blank nodes are mapped to blank nodes, and by leaving out the reference to the graph G' in lines 1 and 2. The definition would then become: Two RDF graphs G and G' are equivalent if there is a bijection M between the sets of nodes of the two graphs, such that: 0. M maps blank nodes to blank nodes 1. M(lit)=lit for all RDF literals lit which are nodes of G 2. M(uri)=uri for all URIs which are nodes of G 3. The triple (s,p,o) is in G iff the triple (M(s),p,M(o)) is in G'. In this way, since M is a bijection between the sets of nodes of the two graphs (and since the sets of URIs, blank nodes and literals are assumed to be pairwise disjoint), it automatically follows that (as is stated in the current definition) M(lit)=lit also holds for each literal lit in G' and that M(uri)=uri also holds for each uri used as a node in G'. Conversely, the current definition implies that blank nodes are mapped to blank nodes. So the two definitions are equivalent. This definition above seems to make more explicit than the current definition what is going on in an equivalence. Moreover, a disadvantage of the current formulation is that, by speaking of M(lit) and M(uri) for nodes lit and uri of G', it is implicitly assumed that these nodes lit and uri are in the domain of the mapping M, i.e., that they are nodes in G. It would be preferable if a spec avoids implicit assumptions. Finally, try to prove with the current definition that equivalence is transitive - one of the crucial properties of an equivalence relation. The implicit assumption just mentioned needs to be invoked two times, in a somewhat awkward way. With the definition above, transitivity follows immediately. Herman ter Horst

Received on Friday, 7 November 2003 07:43:05 UTC