Wreath Products of Forest Algebras, with Applications to Tree Logics
  We use the recently developed theory of forest algebras to find algebraic characterizations of the languages of unranked trees and forests definable in various logics. These include the temporal logics CTL and EF, and first-order logic over the ancestor relation. While the characterizations are in general non-effective, we are able to use them to formulate necessary conditions for definability and provide new proofs that a number of languages are not definable in these logics. 
Mikolaj Bojanczyk (University of Warsaw), Igor Walukiewicz (LaBRI   (Universite de Bordeaux - CNRS)), Howard Straubing (Bost...
cs.LO cs.FL