Next Steps in Propositional Horn Contraction

Amanhã teremos a visita do Ivan Varzinczak, que foi aluno de doutorado do
Andreas Herzig em Toulouse e atualmente trabalha no Instituto Meraka, na África
do Sul. Faremos um seminário extra às 11:30, na sala de reuniões do bloco C.
Seguem os dados abaixo.


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Seminário do Grupo de Lógica, Inteligência Artificial
e Métodos Formais - LIAMF
Seminário Registrado na CPG do IME/USP
Página: http://www.ime.usp.br/~liamf/seminarios/index.html
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Título: Next Steps in Propositional Horn Contraction

Palestrante: Ivan J. Varzinczak

Data:   23/04/2009, 11h30
Local:  Sala de reuniões, bloco C, IME-USP


Abstract: Standard belief contraction assumes an underlying logic containing
full classical propositional logic, but there are good reasons for considering
contraction in less expressive logics. In this paper we focus on Horn logic. In
addition to being of interest in its own right, our choice is motivated by the
use of Horn logic in several areas, including ontology reasoning in description
logics. We consider three versions of contraction: entailment-based and
inconsistency-based contraction (e-contraction and i-contraction, resp.),
introduced by Delgrande for Horn logic, and package contraction (p-contraction),
studied by Fuhrmann and Hansson for the classical case. We show that the
standard basic form of contraction, partial meet, is too strong in the Horn
case. We define more appropriate notions of basic contraction for all three
types above, and provide associated representation results in terms of
postulates. Our results stand in contrast to Delgrande's conjectures that
orderly maxichoice is the appropriate contraction for both e- and i-contraction.
Our interest in p-contraction stems from its relationship with an important
reasoning task in ontological reasoning: repairing the subsumption hierarchy in
EL. This is closely related to p-contraction with sets of basic Horn clauses
(Horn clauses of the form p -> q). We show that this restricted version of
p-contraction can also be represented as i-contraction.

Todos são benvindos.