Almost Periodic Configurations on Linear Cellular Automata

We study computational properties of linear cellular automata on configurations that differ from spatially periodic ones in only finitely many places. It is shown that the degree structure of the orbits of cellular automata is the same on these configurations as on the space of finite configurations. We also show that it is undecidable whether the cellular automaton exhibits complicated behavior on configurations of sufficiently long spatial periods and exhibit cellular automata with undecidable orbits whose orbits on backgrounds of all fixed sizes are decidable.