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We introduce the notion of pre-expansivity for cellular automata (CA): it is the property of being positively expansive on asymptotic pairs of configurations (i.e. configurations that differ in only finitely many positions). Pre-expansivity therefore lies between positive expansivity and pre-injectivity, two important notions of CA theory. We show that there exist one-dimensional pre-expansive CAs which are not positively expansive and they can be chosen reversible (while positive expansivity is impossible for reversible CAs). We provide both linear and non-linear examples. In the one-dimensional setting, we also show that pre-expansivity implies sensitivity to initial conditions in any direction. We show however that no two-dimensional Abelian CA can be pre-expansive. We also consider the finer notion of k -expansivity (positive expansivity over pairs of configurations with exactly k differences) and show examples of linear CA in dimension 2 and on the free group that are k -expansive depending on the value of k , whereas no (positively) expansive CA exists in this setting. [ABSTRACT FROM AUTHOR]

This paper concerns μ -limit sets of cellular automata: sets of configurations made of words whose probability to appear does not vanish with time, starting from an initial μ -random configuration. More precisely, we investigate the computational complexity of these sets and of related decision problems. Main results: first, μ -limit sets can have a Σ 3 0 -hard language, second, they can contain only α -complex configurations, third, any non-trivial property concerning them is at least Π 3 0 -hard. We prove complexity upper bounds, study restrictions of these questions to particular classes of CA, and different types of (non-)convergence of the measure of a word during the evolution. [ABSTRACT FROM AUTHOR]

Abstract: This paper is the first part of a series of two papers dealing with bulking: a quasi-order on cellular automata comparing space–time diagrams up to some rescaling. Bulking is a generalization of grouping taking into account universality phenomena, giving rise to a maximal equivalence class. In the present paper, we discuss the proper components of grouping and study the most general extensions. We identify the most general space–time transforms and give an axiomatization of bulking quasi-order. Finally, we study some properties of intrinsically universal cellular automata obtained by comparing grouping to bulking. [Copyright &y& Elsevier]

Abstract: This paper studies directional dynamics on one-dimensional cellular automata, a formalism previously introduced by the third author. The central idea is to study the dynamical behavior of a cellular automaton through the conjoint action of its global rule (temporal action) and the shift map (spacial action): qualitative behaviors inherited from topological dynamics (equicontinuity, sensitivity, expansivity) are thus considered along arbitrary curves in space–time. The main contributions of the paper concern equicontinuous dynamics which can be connected to the notion of consequences of a word. We show that there is a cellular automaton with an equicontinuous dynamics along a parabola, but which is sensitive along any linear direction. We also show that real numbers that occur as the slope of a limit linear direction with equicontinuous dynamics in some cellular automaton are exactly the computably enumerable numbers. [Copyright &y& Elsevier]

Abstract: This paper is the second part of a series of two papers dealing with bulking: a way to define quasi-order on cellular automata by comparing space-time diagrams up to rescaling. In the present paper, we introduce three notions of simulation between cellular automata and study the quasi-order structures induced by these simulation relations on the whole set of cellular automata. Various aspects of these quasi-orders are considered (induced equivalence relations, maximum elements, induced orders, etc.) providing several formal tools allowing to classify cellular automata. [Copyright &y& Elsevier]

Abstract: The notions of universality and completeness are central in the theories of computation and computational complexity. However, proving lower bounds and necessary conditions remains hard in most cases. In this article, we introduce necessary conditions for a cellular automaton to be “universal”, according to a precise notion of simulation, related both to the dynamics of cellular automata and to their computational power. This notion of simulation relies on simple operations of space–time rescaling and it is intrinsic to the model of cellular automata. Intrinsic universality, the derived notion, is stronger than Turing universality, but more uniform, and easier to define and study. Our approach builds upon the notion of communication complexity, which was primarily designed to study parallel programs, and thus is, as we show in this article, particulary well suited to the study of cellular automata: it allowed us to show, by studying natural problems on the dynamics of cellular automata, that several classes of cellular automata, as well as many natural (elementary) examples, were not intrinsically universal. [ABSTRACT FROM AUTHOR]

Abstract: Proofs of Propositions 6 and 8 of the paper Communication Complexity and Intrinsic Universality in Cellular Automata are formally incorrect. This erratum proves weaker versions of Propositions 6 and 8 and a stronger version of Proposition 9 which are sufficient to get the main results of the paper (Corollary 2) for PREDICTION and INVASION problems. For problem CYCLE, we only prove a weaker version of Corollary 2, essentially replacing a condition of the form ‘’ by ‘’. All other statements of the paper are unaffected. [Copyright &y& Elsevier]