Tom Caylor wrote:

>Actually, in reviewing the definition of Turing machine (it's been over
>2 decades since I studied it) I agree with you.  The Turing machine
>leaves behind a memory of its past through its "writes" to the tape.
>Maybe I don't understand what Wei Dai was saying with his setting of
>the head back to the start of the tape.  In order to get back to the
>exact beginning *state* the Turing Machine would have to be instructed
>to do an inverse of all of the writes it has done and then go back to
>the start of the tape.

But there is not always a unique inverse. There may be two distinct previous 
states A and B which would lead to the same state C on the next step--in 
this case the dynamics are not reversible.


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