Classification of two-dimensional binary cellular automata with respect to surjectivity

Abstract

While the surjectivity of the global map in two-dimensional cellular automata (2D CA) is undecidable in general, in specific cases one can often decide if the rule is surjective or not. We attempt to classify as many 2D CA as possible by using a sequence of tests based on the balance theorem, injectivity of the restriction to finite configurations, as well as permutivity. We introduce the notion of slice permutivity which is shown to imply surjectivity in 2D CA. The tests are applied to 2D binary CA with neighbourhoods consisting of up to five sites, considering all possible contiguous shapes of the neighbourhood. We find that if the size of the neighbourhood is less than five, complete classification of all rules is possible. Among 5-site rules, those with von Neuman neighbourhoods as well as neighbourhoods corresponding to T, V, and Z pentominos can also be completely classified.