Algorithmic Properties of Sigma--definability over Positive Predicate Structures
Margarita Korovina, Oleg V. Kudinov · MIMS EPrints (University of Southampton) · 2011
In this paper we propose a generalisation of the authors p results on semantical characterisation of $\\Sigma$--definability. We prove that over every positive predicate structure a set is $\\Sigma$-definable if and only if it is definable by a disjunction of a recursively enumerable set of existential formulas.