Infinite Time Turing Machines
Joel David Hamkins, Andy Lewis
Abstract
We extend in a natural way the operation of Turing machines to infinite ordinal time, and investigate the resulting supertask theory of computability and decidability on the reals. The resulting computability theory leads to a notion of computation on the reals and concepts of decidability and semi-decidability for sets of reals as well as individual reals. Every Pi11 set, for example, is decidable by such machines, and the semi-decidable sets form a portion of the Delta12 sets. Our oracle concept leads to a notion of relative computability for reals and sets of reals and a rich degree structure, stratified by two natural jump operators.
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