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- Path: sparky!uunet!usc!rpi!ghost.dsi.unimi.it!univ-lyon1.fr!frmop11!barilvm!technion!haifauvm!rsma410
- Organization: University of Haifa.
- Date: Tuesday, 22 Dec 1992 18:27:47 IST
- From: Dan Gordon <RSMA410@HAIFAUVM.BITNET>
- Message-ID: <92357.182747RSMA410@HAIFAUVM.BITNET>
- Newsgroups: comp.theory
- Subject: Re: Automata & Chomsky Hierarchy
- References: <1992Dec15.162545.25338@news.unige.ch>
- Lines: 33
-
- In article <1992Dec15.162545.25338@news.unige.ch>, swann@divsun.unige.ch (SWANN
- Philip) says:
- >
- >In Casti's book "Alternative Realities" there's a conjecture
- >that Wolfram's classification of Cellular Automata into classes
- >can be mapped into Chomsky's Hierarchy of Formal Languages. He
- >cites Wolfram on this. Is there any recent work on the subject?
-
- Cellular automata, in the unrestricted sense, have long ago been
- proved to be computation universal, i.e., they can simulate any
- Turing machine - see A.R. Smith III, "Simple computation-universal
- cellular spaces, JACM 18(1971), 339-353. This means, of course,
- that they can also simulate any of the following: finite automata,
- pushdown automata, and linear-bounded automata. So cellular
- automata can be "mapped" to the Chomsky hierarchy (if this is the
- intended meaning of "mapping into the Chomsky hierarchy").
-
- Wolfram, in "Statistical mechanics of cellular automata," Rev.
- Modern Physics 55(1983),601-644, studies so-called _totalistic_
- cellular automata. In these automata, each state is considered
- as a non-negative integer, and the state transition function
- depends only on the sum of the states of a cell's neighborhood
- (the sum includes the cell's own state). Wolfram conjectured
- that these automata are also computation-universal.
-
- This conjecture about totalistic cellular automata was proved
- true in: D. Gordon, "On the computational power of totalistic
- cellular automata," Math. Systems Theory 20(1987), 43-52.
- It is shown there that totalistic cellular automata can simulate
- an arbitrary Turing machine in linear time - actually, twice
- real time. Needless to say, I was somewhat surprised to learn
- from the above posting that this problem is still regarded by
- some as a conjecture.
-