The expressive power of revised Datalog on problems with closure   properties
  In this paper, we study the expressive power of revised Datalog on the problems that are closed under substructures. We show that revised Datalog cannot define all the problems that are in PTIME and closed under substructures. As a corollary, LFP cannot define all the extension-closed problems that are in PTIME. 
Shiguang Feng
cs.LO