Cellular automata with almost periodic initial conditions 
A. Hof and O. Knill
October 1994 (revised March 1995)
This paper appeared in the journal Nonlinearity, 8 p. 477491, 1995 MR 96g:58093 
1991 Mathematics Subject Classification: 82C20, 54H20, 58Fxx 
Abstract 
Cellular automata are dynamical systems on the compact metric space of subshifts. They leave many classes of subshifts invariant. Here we show that cellular automata leave `circle subshifts' invariant. These are the strictly ergodic subshifts of obtained by a circle sequence , where J is a finite union of halfopen intervals. For such initial conditions, the evolution of the whole infinite configuration can be computed by evolving the finitely many parameters defining the set J. Moreover, many macroscopic quantities can be computed exactly for the infinite system. We illustrate that in one dimension by rule 18 and in two dimensions by the Game of Life. The ideas also apply to cellular automata acting on . This we illustrate by the HPP model, a lattice gas automaton with N=16. 


