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Re: Detecting cycles in OWL

From: Doerthe Arndt <doerthe.arndt@ugent.be>
Date: Tue, 5 May 2020 12:56:35 +0200
To: semantic-web@w3.org
Message-ID: <2f0ded7f-c0fc-0c02-163b-76051933c4fc@ugent.be>
Dear Enrico,

>
>> Regarding Enrico’s point: since N3 rules are seen as logical 
>> implications, one can reason over them, e.g., use them as part of 
>> logical conditions, or even infer new implications during reasoning.
>
> I understand that N3 rules allow existential in the head, and 
> recursive rules (i.e., arbitrary TGDs).
> But please tell me whether my understanding is true or if there is 
> some restriction on this.
> This makes entailment in the language undecidable. Also the chase 
> would be non-terminating.
>
You are right that N3 allows existentials in the head of a rule and that 
the language is not decidable. It is however still possible to put 
restrictions on the rules themselves (as it is also done for existential 
Datalog, there are too many publications about it to add a reference) or 
to implement different reasoning strategies which are incomplete for 
some cases but guarantee termination (this is done in EYE [1]).


>> On the open world topic, N3 features scoped negation as failure 
>> (SNAF) which allows checking whether a document / formula holds or 
>> allows deriving a given fact at a given point in time. While N3 
>> implements the open world assumption, it allows for a monotonic 
>> negation as failure - “closing” the world to an extent that is often 
>> useful in practice.
>
> That's interesting. I suspect that you have carefully chosen a 
> restriction when a (sort of) minimal model exists.
> Am I right? If yes, then this is basically "closed world".
> But, again, tell me whether I;m wrong.

Scoped negation as failure in N3 is not formalised yet. We  plan to 
follow exiting publications [2,3] but adjust them to our applications. 
We currently discuss this adjustment and also the concrete way to 
express that in N3 (so suggestions are welcome).

The basic idea of scoped negation as failure is that you keep the 
monotonicity by explicitly mentioning the scope of your negation. So, 
you have a "closed sub-world" instead of a "closed world".


>
> BTW: is there a spec with a logic-based semantics?

The spec is not ready yet but we are working on it and hope to be able 
to release a first version soon. If you are interested, I point you to 
my paper about N3's semantic [4] where many aspects are already covered 
(as mentioned above, the negation is still missing).

Kind regards,
Doerthe


[1] http://eulersharp.sourceforge.net/

[2] https://link.springer.com/content/pdf/10.1007/11762256_26.pdf

[3] https://aic.ai.wu.ac.at/~polleres/publications/poll-2006.pdf

[4] https://biblio.ugent.be/publication/8627430/file/8632611.pdf


-- 
Dörthe Arndt
Researcher Semantic Web
imec - Ghent University - IDLab | Faculty of Engineering and Architecture | Department of Electronics and Information Systems
Technologiepark-Zwijnaarde 122, 9052 Ghent, Belgium
t: +32 9 331 49 59 | e: doerthe.arndt@ugent.be
Received on Tuesday, 5 May 2020 10:56:52 UTC

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