union of graphs [soolved with a minor issue]

As a previous email of mine indicated, we can consider my objection to 
the definition of entailments with sets moot and move on.

The simple solution that Peter proposed is indeed very good.



There subsists residue of the previous design:

"""
In order to capture the full meaning of graphs sharing a blank node, it 
is necessary to consider the union graph containing all the triples 
which contain the blank node.
"""

This suggests that union is the only logical operation that makes sense. 
People *can* consider the merge instead.

With union, there are strange properties of RDF graphs:

if X is equivalent to Y, it would seem logical that {X, Y} is equivalent 
to X (X being equivalent to Y seems to intuitively mean that Y does not 
bring any information in addition to X, so the set {X, Y} should be 
equivalent to Y. But this is not the case if you definition the truth of 
{X, Y} as the union of X and Y.

E.g., if b1 and b2 are two different blank nodes, then:

(b1, <p>, b2)  is equivalent to  (b2, <p>, b1).

Clearly, there is no gain of information when you know one or the other.
Yet, the union says something more.

This also means that one should never copy a graph using different 
bnodes, because they really lose information.

If I copy the first graph above, using bnodes b3, b4 instead of b1 and 
b2, leading to (b3, <p>, b4), then I can't compute the inferences that I 
would have had with the original graphs. Yet, I only made 
truth-preserving operations. How can this be?

To summarise, it is possible to apply a sequence of presumably 
"truth-preserving" operations and end up doing something that is not 
truth-preserving. This is simply because union of graphs is not 
truth-preserving.
-- 
Antoine Zimmermann
ISCOD / LSTI - Institut Henri Fayol
École Nationale Supérieure des Mines de Saint-Étienne
158 cours Fauriel
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 Tuesday, 25 June 2013 15:58:38 UTC