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Re: SKOS Comment: skos:broader as subproperty of broaderTransitive seems backwards

From: Sean Bechhofer <sean.bechhofer@manchester.ac.uk>
Date: Mon, 1 Dec 2008 10:18:26 +0000
Message-Id: <809D676D-94B6-4CB5-8E70-3DF5E29A60A8@manchester.ac.uk>
Cc: "public-swd-wg@w3.org" <public-swd-wg@w3.org>
To: "Booth, David (HP Software - Boston)" <dbooth@hp.com>

On 1 Dec 2008, at 07:50, Booth, David (HP Software - Boston) wrote:

> In
> http://www.w3.org/TR/2008/WD-skos-reference-20080829/#L2413
> it says that "skos:broader is not a transitive property", but  
> "skos:broader is a sub-property of skos:broaderTransitive, which is  
> a transitive property".  Isn't this backwards?
> By the entailment rules for rdfs:subPropertyOf
> http://www.w3.org/TR/rdf-mt/#rulerdfs7
> if:
>         skos:broader rdfs:subPropertyOf skos:broaderTransitive .
>         uuu skos:broader yyy .
> then necessarily:
>         uuu skos:broaderTransitive yyy .


A subproperty of a transitive property is not necessarily transitive.  
As you rightly point out, if I have

xxx skos:broader yyy

then I can infer

xxx skos:broaderTransitive yyy

However, this doesn't mean skos:broader is transitive. If a relation  
R is transitive, it means that from xRY and yRz, we can infer xRz. So  
the situation with broader is as follows.

If I have

xxx skos:broader yyy
yyy skos:broader zzz

then I can infer (through subproperties) that

xxx skos:broaderTransitive yyy
yyy skos:broaderTransitive zzz

Now, due to the transitivity of broaderTransitive, I can infer

xxx skos:broaderTransitive zzz

This *doesn't* mean that xxx skos:broader zzz, so I don't  
(necessarily) have transitivity of skos:broader.

The WG spent some time discussing this. We believe that there are  
situations where we do not necessarily want skos:broader to be  
transitive, but the pattern we have used allows us to /query/ across  
something which includes the transitive closure of skos:broader,  
without us having to assert that skos:broader is itself transitive.  
We believe that this is the desirable situation.

Hope that helps.


Sean Bechhofer
School of Computer Science
University of Manchester
Received on Monday, 1 December 2008 10:20:42 UTC

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