Generalized linear cellular automata in groups and difference Galois theory II

Abstract

In the first part of this series the $σ$-ring spanned by the periodic solutions of a generalized linear cellular automaton in a discrete group was shown to be Hopf--Galois, with pro-algebraic Galois group, while a Galois theory for the $σ$-ring spanned by the finite support solutions was left open, with parameterized difference Galois theory suggested as the tool. We apply that tool to automata over $\mathbb Z$. The Fourier transform turns the automaton into a rank one equation $σ(\hat x)=\hatα\hat x$ over a field on which $δ=z\,d/dz$ acts, and we compute, and algorithmically decide, its parameterized Galois group: for genuine automata with symbols rational in the time variable it is either $\mathbb{G}_m(\mathbb C)$ or $\mathbb{G}_m$, the dividing line being separability of the symbol, $\hatα(z,t)=γ(z)ψ(t)$. Finally, the parameterized group in the time direction decides holonomy in the space direction; as a corollary, the Stirling numbers of the first kind are not spatially holonomic.