Duality and Regular Languages

In this work we analyse the class of regular languages from an algebraic point of view. There exist interesting relations between regular languages and algebraic structures like monoids, Boolean algebras, lattices and varieties of algebras. In particular we focuse on finite monoids. In fact, a language L is regular if there exists a homomorphism from the free monoid A* of the words onto a finite monoid in such a way that L is the inverse image of a subset of the monoid. Monoids can be used to define a metric over A*, whose completion defines the Boolean space of profinite words, where a profinite word is a Cauchy sequence of words of A*. The Priestly duality between the Boolean space of profinite words and the Boolean algebra of its clopen subsets can be used to prove the following theorems: 1. A class of languages is a Boolean algebra of languages iff the class can be defined by a set of profinite equations; 2. A language is regular if its closure is clopen in the Boolean space of profinite words; 3. String concatenation is the dual operation of the residuals of right/left product on the Boolean algebra of regular languages.