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- From: swann@divsun.unige.ch (SWANN philip)
- Subject: Re: Automata & Chomsky Hierarchy
- Message-ID: <1992Dec30.084134.833@news.unige.ch>
- Sender: usenet@news.unige.ch
- Organization: University of Geneva, Switzerland
- References: <1992Dec15.162545.25338@news.unige.ch> <92357.182747RSMA410@HAIFAUVM.BITNET>
- Date: Wed, 30 Dec 1992 08:41:34 GMT
- Lines: 61
-
- In article <92357.182747RSMA410@HAIFAUVM.BITNET>, Dan Gordon <RSMA410@HAIFAUVM.BITNET> writes:
- > 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.
-
- Sorry, my question was vague. I've now found the original
- reference.... The classification is of the limiting behavior of
- *one-dimensional* automata as:
-
- A. The initial pattern disappears
- B. The initial pattern evolves to a fixed finite size
- C. The initial pattern grows indefinitely at a fixed rate
- D. The initial pattern grows and contracts with time
-
- Casti then conjectures the following theorem:
-
- "Class A and B cellular automata have limit sets that form
- regular languages. For most Class C and D automata, the
- regular language complexity increases monotonically with
- time so that the configurations obtained in the limit usually
- do not form a regular language. Instead, the limit sets for
- Class C automata appear to form context-sensitive languages,
- whereas those for Class D automata seem to correspond to
- general, unrestricted languages" (p. 71)
-
- It's not clear from the references where Casti gets this
- from: he refers to books by Davis & Weyuker (1983) and
- Eilenberg (1974-76) and to a paper by Wolfram (1984) "Computation
- Theory of CA's" Comm. Math. Physics 96, pp. 15-57
-
- Philip Swann
-