Petr Kurka Alejandro Maass Stability of subshifts in cellular automata. 143-155 2002 52 Fundam. Inform. 1-3 http://iospress.metapress.com/openurl.asp?genre=article&issn=0169-2968&volume=52&issue=1&spage=143 db/journals/fuin/fuin52.html#KurkaM02