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Logic in Computer Science, Symposium on (2009)
Los Angeles, California
Aug. 11, 2009 to Aug. 14, 2009
ISSN: 1043-6871
ISBN: 978-0-7695-3746-7
pp: 255-263
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.

M. Bojanczyk, I. Walukiewicz and H. Straubing, "Wreath Products of Forest Algebras, with Applications to Tree Logics," Logic in Computer Science, Symposium on(LICS), Los Angeles, California, 2009, pp. 255-263.
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