Summary
Every deterministic t(n)-time bounded multitape Turing machine can be simulated by an alternating t(n) loglog t(n)/log t(n)-time bounded Turing machine.
If the depth of every directed acyclic graph with n edges can be reduced to log n by removing only o(n) edges, then in linear time nondeterministic multitape Turing machines can recognize mor languages than deterministic multitape Turing machines. For some graphs reduction of the depth to log n requires the removal of Ω(n/loglog n) edges. A graph theoretic condition is given, which implies that obliviousness reduces the power of multitape Turing machines.
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Part of this research was done while the author was visiting the ‘Laboratoire de recherches en informatique de l'université de Paris sud’ under DAAD-grant 311-f-HSLA-soe
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Paul, W., Reischuk, R. On alternation II. Acta Informatica 14, 391–403 (1980). https://doi.org/10.1007/BF00286494
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DOI: https://doi.org/10.1007/BF00286494