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Expressiveness of OWL

From: Graham Klyne <gk@ninebynine.org>
Date: Wed, 28 Apr 2004 19:10:09 +0100
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To: www-rdf-logic@w3.org

I've been doing a bit more reading about Description Logics, and I come to 
a conclusion that I haven't noted stated anywhere in the OWL specifications...

(1) No "role composition" in OWL

The Description Logic literature makes mention of a "role constructor" 
called 'compose'.  I find no equivalent property in OWL;  I don't think 
this is surprising, as DL literature indicates that DLs containing 
'compose' are undecidable, but I'd like to be sure I'm not overlooking 

(2) Limited expressiveness of OWL-based inferences

OWL expressiveness limited to FOL expressions with just monadic and dyadic 
predicates and no more than two variables.  Assuming absence of an 
equivalent to 'compose' in OWL, this result is noted in a couple of places, 
most clearly among those I surveyed in [1].

What does this all mean?  Thinking in terms of Horn Clause rules:
    A1,A2,...,An => B
    ~A1 \/ ~A2 \/ ... \/ ~An \/ B

(The results in [1] are based on conjunctive forms, but I am guessing that 
this is a dual result that can be obtained by renormalization.)

This suggests that OWL-based inference can handle a maximum of two 
variables between the antecedent and concequent of a rule, so something like:

    :a :parentOf :b .
    :b :parentOf :c .
    :a :grandParentOf :c .

Is beyond the scope of an OWL based reasoner to infer.

Is this right, or am I missing something?


[1] Alex Bordida,
"On the Relative Expressiveness of Description Logics and Predicate Logics"

Graham Klyne
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Received on Wednesday, 28 April 2004 14:50:30 UTC

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