Copyless Cost-Register Automata: Structure, Expressiveness, and Closure Properties

TL;DR

This paper analyzes the structure and capabilities of copyless cost-register automata, revealing their limitations and proposing a restricted, more robust subclass that is closed under reverse and other operations.

Contribution

It characterizes the expressiveness and closure properties of copyless CRA and introduces a new restricted class with improved closure properties.

Findings

Copyless CRA are less expressive than weighted automata.

They are not closed under reverse operation.

A new restricted class is proposed with better closure properties.

Abstract

Cost register automata (CRA) and its subclass, copyless CRA, were recently proposed by Alur et al. as a new model for computing functions over strings. We study some structural properties, expressiveness, and closure properties of copyless CRA. We show that copyless CRA are strictly less expressive than weighted automata and are not closed under reverse operation. To find a better class we impose restrictions on copyless CRA, which ends successfully with a new robust computational model that is closed under reverse and other extensions.