Decomposition of Automata recognizing Ideals

Abstract

Minimizing the size of finite automata is a fundamental problem in theoretical computer science. Beyond standard minimization, further reductions can be achieved by decomposing an automaton into smaller components whose languages combine via intersection or union to recover the original language. However, in general, no polynomial-time algorithm is known for computing such decompositions. In this paper, we focus on automata that recognize ideals, that is, languages at level 1/2 in the Straubing-Th\'erien hierarchy. Equivalently, these languages are expressible as a finite union of languages of the form $\Sigma^*a_1\Sigma^*\dots\Sigma^*a_n\Sigma^*$ where $\Sigma$ is an alphabet and $a_i$ are letters of $\Sigma$. We show that the two problems of deciding whether such a language can be decomposed into an intersection or a union of smaller automata are decidable in NL. Moreover, we provide a polynomial-time algorithm that computes a decomposition into an intersection, if one exists, while ensuring that the resulting components also recognize ideal languages.