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Oracle machine

Definition

In complexity theory and computability theory, an oracle machine is an abstract machine that can query a black box called an oracle, which is able to give an answer to any instance of a certain problem ⁠⁠ in a single operation. The problem ⁠⁠ can be of any complexity class, or it can even be an undecidable problem such as the halting problem. If another problem ⁠⁠ is reducible to ⁠⁠ in polynomial time, then the oracle machine can solve ⁠⁠ in polynomial time; one can say that ⁠⁠ is in the relativized complexity class ⁠⁠. Other relativized complexity classes such as ⁠⁠ can be defined analogously.

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