The Solvability of the Halting Problem for 2-State Post Machines

Authors:
Stål Aanderaa;Patrick C. Fischer
Affiliations:
Oslo University, Oslo, Norway;Cornell University, Ithaca, New York
Venue:
Journal of the ACM (JACM)
Year:
1967

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Abstract

A Post machine is a Turing machine which cannot both write and move on the same machine step. It is shown that the halting problem for the class of 2-state Post machines is solvable. Thus, there can be no universal 2-state Post machine. This is in contrast with the result of Shannon that there exist universal 2-state Turing machines when the machines are capable of both writing and moving on the same step.