PRODUCTS OF TREE LANGUAGES
Abstract
Sets of terms of type τ are called tree languages. The tree language product is an
important operation defined on sets of tree languages which maps recognizable
tree languages to recognizable tree languages. This tree language product can
be described by the superposition of sets of terms. Based on the superposition
operation we define a binary associative operation. In the theory of tree languages
the product of languages is called the z-product. The aim of this paper is to study
properties of the arising semigroups and its subsemigroups. We are especially
interested in idempotent and regular elements, Green’s relations L and R, in
constant, left-zero and right-zero subsemigroups and in rectangular bands. Since
the set of all recognizable tree languages of a given type is closed under the
language product, the set of all recognizable tree languages forms a subsemigroup
of the semigroup of all tree languages which contains the semigroup of all finite
tree languages of the given type. Recognizable tree languages can be generated by
regular tree grammars. We characterize all idempotent elements in the semigroup
of all recognizable tree languages of type τ by properties of regular grammars.
The iteration of the language product plays the role of the Kleene-∗-operation in
the theory of formal languages and is one of the regular operations.
Keywords: Tree languages; product of tree languages; regular elements;
recognizable tree languages; Green’s relations; right-zero semigroups; leftzero
semigroups; rectangular bands.
AMS Subject Classification: 08C99, 20M17