Kleene Theorems for Lasso Languages and ω-Languages

Automata operating on pairs of words were introduced as an alternative way of capturing acceptance of regular ω-languages. Families of DFAs and lasso automata operating on such pairs followed, giving rise to minimisation algorithms, a Myhill-Nerode theorem and language learning algorithms. Yet Kleene theorems for such a well-established class are still missing. Here, we introduce rational lasso languages and expressions, show a Kleene theorem for lasso languages and explore the connection between rational lasso and ω-expressions, which yields a Kleene theorem for ω-languages with respect to saturated lasso automata. For one direction of the Kleene theorems, we also provide a Brzozowski construction for lasso automata from rational lasso expressions.