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CommentResponse:EF

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Enrico, thanks for your additional explanations and your comments in response to Simon Reinhardt in

Although they are not, strictly speaking, comments on the current working drafts, the working group felt that we should also reply to you. We apologise for the delay.

Simon wrote: 

>> Maybe it's worth investigating at some point how much of OWL 2 RL 
>> could be implemented with pure SPARQL - and what extensions would 
>> be needed to add the missing bits. But that's just something to keep in 
>> mind for the future. :-)

to which you replied: 

> If you fix the entailment regime to RDFS, then very little of OWL2 can be
> encoded in SPARQL, since it is has been shown that the computational 
> complexities diverge too much. There are also simple counter-examples
> showing that it does not make sense to have an OWL2 entailment regime in
> SPARQL, since you would get unsound results (wrt OWL2 semantics) very
> easily. So, really, SPARQL can hardly go beyond RDFS.

and 

> Ooops, sorry: I realise just now that you were talking about OWL2 *RL*.
> I don't have a full picture of OWL2 RL, but: if OWL2 RL allows for recursive
> rules, then again SPARQL can not encode it due to a data complexity argument
> - linear time lower bound for recursive rules as opposed to AC0 (sub-linear
> and sub-logspace) upper bound for SPARQL.

For the OWL entailment regimes we do not envisage an encoding into SPARQL queries that are then evaluated under simple entailment semantics. The current working draft includes an OWL Direct Semantics entailment regime that hopefully clarifies this. The working draft does not yet define an entailment regime for OWL RDF-Based Semantics (which is used for the OWL 2 RL profile), but that is envisaged for a future working draft.

Regards, Birte Glimm on behalf of the SPARQL Working Group

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