Discrete Mathematics & Theoretical Computer Science, Vol 14, No 1 (2012)

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The Join of the Varieties of R-trivial and L-trivial Monoids via Combinatorics on Words

Manfred Kufleitner, Alexander Lauser


The join of two varieties is the smallest variety containing both. In finite semigroup theory, the varieties of R-trivial and L-trivial monoids are two of the most prominent classes of finite monoids. Their join is known to be decidable due to a result of Almeida and Azevedo. In this paper, we give a new proof for Almeida and Azevedo's effective characterization of the join of R-trivial and L-trivial monoids. This characterization is a single identity of ω-terms using three variables.

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