Collisions and their Catenations: Ultimately Periodic Tilings of the Plane

Motivated by the study of cellular automata algorithmic and dynamics, we investigate an extension of ultimately periodic words to twodimensional infinite words: collisions. A natural composition operation on tilings leads to a catenation operation on collisions. By existence of aperiodic tile sets, ultimately periodic tilings of the plane cannot generate all possible tilings but exhibit some useful properties of their one-dimensional counterparts: ultimately periodic tilings are recursive, very regular, and tiling constraints are easy to preserve by catenation. We show that, for a given catenation scheme of finitely many collisions, the generated set of collisions is