Process algebra
Automata, Languages, and Machines
Automata, Languages, and Machines
Varieties Of Formal Languages
On Logical Descriptions of Regular Languages
LATIN '02 Proceedings of the 5th Latin American Symposium on Theoretical Informatics
Shuffle on positive varieties of languages
Theoretical Computer Science
A Robust Class of Regular Languages
MFCS '08 Proceedings of the 33rd international symposium on Mathematical Foundations of Computer Science
On the Shuffle of Star-Free Languages
Fundamenta Informaticae - Words, Graphs, Automata, and Languages; Special Issue Honoring the 60th Birthday of Professor Tero Harju
A Hierarchy of Languages with Catenation and Shuffle
Fundamenta Informaticae - Concurrency, Specification and Programming
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There is an increasing interest in the shuffle product on formal languages, mainly because it is a standard tool for modeling process algebras. It still remains a mysterious operation on regular languages. Antonio Restivo proposed as a challenge to characterize the smallest class of languages containing the singletons and closed under Boolean operations, product and shuffle. This problem is still widely open, but we present some partial results on it. We also study some other smaller classes, including the smallest class containing the languages composed of a single word of length 2 which is closed under Boolean operations and shuffle by a letter (resp. shuffle by a letter and by the star of a letter). The proof techniques have both an algebraic and a combinatorial flavor.