Tiling-Recognizable Two-Dimensional Languages: From Non-Determinism to Determinism through Unambiguity

TL;DR

This paper explores the hierarchy of two-dimensional tiling recognizable languages, introducing line-unambiguity to bridge non-determinism and determinism, and analyzing their theoretical properties and limitations.

Contribution

It defines line-unambiguous tiling recognizable languages and establishes a hierarchy within REC, connecting unambiguity with determinism in two-dimensional languages.

Findings

REC is not closed under complementation.

Membership problem for REC is NP-complete.

Line-unambiguity introduces a hierarchy of deterministic notions.

Abstract

Tiling recognizable two-dimensional languages, also known as REC, generalize recognizable string languages to two dimensions and share with them several theoretical properties. Nevertheless REC is not closed under complementation and the membership problem is NP-complete. This implies that this family REC is intrinsically non-deterministic. The natural and immediate definition of unambiguity corresponds to a family UREC of languages that is strictly contained in REC. On the other hand this definition of unambiguity leads to an undecidability result and therefore it cannot correspond to any deterministic notion. We introduce the notion of line-unambiguous tiling recognizable languages and prove that it corresponds or somehow naturally introduces different notions of determin- ism that define a hierarchy inside REC.