Syntactic Complexity of R- and J-Trivial Regular Languages

The syntactic complexity of a regular language is the cardinality of its syntactic semigroup. The syntactic complexity of a subclass of the class of regular languages is the maximal syntactic complexity of languages in that class, taken as a function of the state complexity n of these languages. We study the syntactic complexity of Rand J -trivial regular languages, and prove that n! and ⌊e(n − 1)!⌋ are tight upper bounds for these languages, respectively. We also prove that 2 is the tight upper bound on the state complexity of reversal of J -trivial regular languages.