Decision Problems for Recognizable Languages of Infinite Pictures

TL;DR

This paper explores the high undecidability and set-theoretic independence of decision problems related to recognizable languages of infinite two-dimensional words (infinite pictures), extending classical automata theory results.

Contribution

It establishes new high-level undecidability results and set-theoretic independence for properties of B"uchi-recognizable languages of infinite pictures.

Findings

Determining unambiguity is $oldsymbol{ ext{Pi}}_2^1$-complete.

Cardinality problems are $D_2(oldsymbol{ ext{Sigma}}_1^1)$-complete and $oldsymbol{ ext{Sigma}}_1^1$-complete.

Complement cardinality problems depend on the set-theoretic model and are in complex projective classes.

Abstract

Altenbernd, Thomas and W\"ohrle have considered in [ATW02] acceptance of languages of infinite two-dimensional words (infinite pictures) by finite tiling systems, with the usual acceptance conditions, such as the B\"uchi and Muller ones, firstly used for infinite words. Many classical decision problems are studied in formal language theory and in automata theory and arise now naturally about recognizable languages of infinite pictures. We first review in this paper some recent results of [Fin09b] where we gave the exact degree of numerous undecidable problems for B\"uchi-recognizable languages of infinite pictures, which are actually located at the first or at the second level of the analytical hierarchy, and "highly undecidable". Then we prove here some more (high) undecidability results. We first show that it is $\Pi_2^1$-complete to determine whether a given B\"uchi-recognizable languages of infinite pictures is unambiguous. Then we investigate cardinality problems. Using recent results of [FL09], we prove that it is $D_2(\Sigma_1^1)$-complete to determine whether a given B\"uchi-recognizable language of infinite pictures is countably infinite, and that it is $\Sigma_1^1$-complete to determine whether a given B\"uchi-recognizable language of infinite pictures is uncountable. Next we consider complements of recognizable languages of infinite pictures. Using some results of Set Theory, we show that the cardinality of the complement of a B\"uchi-recognizable language of infinite pictures may depend on the model of the axiomatic system ZFC. We prove that the problem to determine whether the complement of a given B\"uchi-recognizable language of infinite pictures is countable (respectively, uncountable) is in the class $\Sigma_3^1 \setminus (\Pi_2^1 \cup \Sigma_2^1)$ (respectively, in the class $\Pi_3^1 \setminus (\Pi_2^1 \cup \Sigma_2^1)$).