CSSE AND CARMA SEMINAR Speaker: Prabhu Manyem Title: Expressibility at the machine level versus structure level for ESO universal Horn Logic: Easier done than said Location: Room EF122, Engineering Building EF (Callaghan Campus) The University of Newcastle Time and Date: 1:00 pm, Wed, 2nd Apr 2014 Abstract: We show that ESO universal Horn logic (existential second logic where the first order part is a universal Horn formula) is insufficient to capture P, the class of problems decidable in polynomial time. This is true in the presence of a successor relation in the input vocabulary. We provide two proofs -- one based on reduced products of two structures, and another based on approximability theory (the second proof is under the assumption that P is not the same as NP). The difference between the results here and those in (Graedel 1991) is due to the fact that the expressions this talk deals with are at the "structure level", whereas the expressions in (Graedel 1991) are at the "machine level" since they encode machine computations -- a case of "Easier DONE than SAID". [Permanent link]