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SUMMARY:Abhisekh Sankaran (University of Cambridge)
DTSTART:20201111T140000Z
DTEND:20201111T150000Z
DTSTAMP:20260423T052500Z
UID:OWLS/16
DESCRIPTION:Title: <a href="https://researchseminars.org/talk/OWLS/16/">Ex
 tension preservation in the finite and prefix classes of first order logic
 </a>\nby Abhisekh Sankaran (University of Cambridge) as part of Online Wor
 ldwide Seminar on Logic and Semantics (OWLS)\n\n\nAbstract\nIt is well kno
 wn that the classic Łoś-Tarski preservation theorem fails in the finite:
  there are first-order definable classes of finite structures closed under
  extensions which are not definable (in the finite) in the existential fra
 gment of first-order logic. We strengthen this by constructing for every n
 \, first-order definable classes of finite structures closed under extensi
 ons which are not definable with n quantifier alternations. The classes we
  construct are definable in the extension of Datalog with negation and ind
 eed in the existential fragment of transitive-closure logic. This answers 
 negatively an open question posed by Rosen and Weinstein.\n
LOCATION:https://researchseminars.org/talk/OWLS/16/
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