Closure Properties of Subregular Languages Under Operations

Abstract

A class of languages is closed under a given operation if the resulting language belongs to this class whenever the operands belong to it. We examine the closure properties of various subclasses of regular languages under basic operations of intersection, union, concatenation and power, positive closure and star, reversal, and complementation. We consider the following classes: definite languages and their variants (left ideal, finitely generated left ideal, symmetric definite, generalized definite and combinational), two-sided comets and their variants comets and stars, and the classes of singleton, finite, ordered, star-free, and power-separating languages. We also give an overview about subclasses of convex languages (classes of ideal, free, and closed languages), union-free languages, and group languages. We summarize some inclusion relations between these classes. Subsequently, for all pairs of a class and an operation, we provide an answer whether this class is closed under this operation or not.