Star-free languages are Church–Rosser congruential

Volker Diekert, Manfred Kufleitner, Pascal Weil
2012 Theoretical Computer Science  
The class of Church-Rosser congruential languages has been introduced by McNaughton, Narendran, and Otto in 1988. A language L is Church-Rosser congruential (belongs to CRCL), if there is a finite, confluent, and length-reducing semi-Thue system S such that L is a finite union of congruence classes modulo S. To date, it is still open whether every regular language is in CRCL. In this paper, we show that every star-free language is in CRCL. In fact, we prove a stronger statement: For every
more » ... ree language L there exists a finite, confluent, and subword-reducing semi-Thue system S such that the total number of congruence classes modulo S is finite and such that L is a union of congruence classes modulo S. The construction turns out to be effective.
doi:10.1016/j.tcs.2012.01.028 fatcat:qojk42tvc5cado275mrhsryiri