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Oracle machine
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==Oracles and halting problems== A machine with an oracle for the [[halting problem]] can determine whether particular Turing machines will halt on particular inputs, but it cannot determine, in general, whether machines equivalent to itself will halt. This creates a hierarchy of machines, each with a more powerful halting oracle and an even harder halting problem. This hierarchy of machines can be used to define the ''[[arithmetical hierarchy]]''.{{sfn|Börger|1989|p=141}}
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