Commutative images of rational languages and the Abelian kernel of a monoid

Centro de Matemática, Universidade do Porto P. Gomes Teixeira, 4099-002 Porto, Portugal; mdelgado@fc.up.pt.

Received: 6 March 2001
Accepted: 1 March 2002

Abstract

Natural algorithms to compute rational expressions for recognizable languages, even those which work well in practice, may produce very long expressions. So, aiming towards the computation of the commutative image of a recognizable language, one should avoid passing through an expression produced this way. We modify here one of those algorithms in order to compute directly a semilinear expression for the commutative image of a recognizable language. We also give a second modification of the algorithm which allows the direct computation of the closure in the profinite topology of the commutative image. As an application, we give a modification of an algorithm for computing the Abelian kernel of a finite monoid obtained by the author in 1998 which is much more efficient in practice.