A counterexample to a conjecture concerning concatenation hierarchies

Jorge Almeida, Ondřej Klíma
2009 Information Processing Letters  
We give a counterexample to the conjecture which was originally formulated by Straubing in 1986 concerning a certain algebraic characterization of regular languages of level 2 in the Straubing-Thérien concatenation hierarchy of star-free languages.
doi:10.1016/j.ipl.2009.09.011 fatcat:zt3i56v3fveb3idvasjszlaqs4