Jorge Almeida, Ondrej Klima. New decidable upper bound of the second level in the Straubing-Therien concatenation hierarchy of star-free languages.
Discrete Mathematics and Theoretical Computer Science, DMTCS, 2010, Vol. 12 no. 4 (4), pp.41-58.
⟨10.46298/dmtcs.490⟩.
⟨hal-00990443⟩