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arXiv · 2603.23570

No weakly factor-universal cellular automaton

Maja Gwozdz
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Abstract

Hochman asked whether there exists a cellular automaton $F$ such that every cellular automaton is a factor of $F$ in the dynamical sense. In particular, we do not require the factor map to commute with the spatial shifts. We show that no such cellular automaton exists. More generally, if $F$ weakly factors onto the radius-zero $q$-clock automaton $C_q^{(k)}$, then every periodic point of $F$ has period divisible by $q$. For a cellular automaton $F:A^{\mathbb Z^d}\to A^{\mathbb Z^d}$, define $\varphi_F:A\to A$ by $F(\underline a)=\underline{\varphi_F(a)}$, and let $g_F$ be the greatest common divisor of the cycle lengths of $\varphi_F$. We prove that if $C_q^{(k)}$ is a weak factor of $F$, then $q\mid g_F$ holds. It follows that the action of $F$ on constant configurations yields an explicit divisibility obstruction to clock weak factors.

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Maja Gwozdz. 2026-03-24. No weakly factor-universal cellular automaton. https://arxiv.org/abs/2603.23570

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