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Published online by Cambridge University Press: 15 August 2002
If we consider words over the alphabet which is the set of all elementsof a semigroup S, then such a word determines an element of S:the product of the letters of the word. S is strongly locally testable if whenever two words over thealphabet S have the same factors of a fixed length k, then the products of the letters of these words are equal. We had previously proved [19] thatthe syntactic semigroup of a rational language L is strongly locally testable if and only ifL is both locally and piecewise testable. We characterize in this paper the variety of strongly locally testable semigroups with commuting idempotents and, using the theory of implicit operations on avariety of semigroups, we derive an elementary combinatorial description of the related variety of languages.