Oracle machine
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In complexity theory and computability theory, an oracle machine is an abstract machine used to study decision problems. It can be visualized as a Turing machine with a black box, called oracle, which is able to decide certain decision problems in a single step. The problem can be of any complexity class. Even undecidable problems, like the halting problem, can be used.
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Definition
An oracle machine is a Turing machine connected to an oracle. The Turing machine can write on its own tape an input for the oracle, then tell the oracle to execute. In a single step, the oracle computes its function, erases its input, and writes its output to the tape. Sometimes the Turing machine is described as having two tapes, one of which is reserved for oracle inputs and one for outputs.
Complexity classes of oracle machines
The complexity class of decision problems solvable by an algorithm in class A with an oracle for a problem in class B is written A^{B}. For example, the class of problems solvable in polynomial time by a deterministic Turing machine with an oracle for a problem in NP is P^{NP}. (This is also the class of problems reducible by polynomialtime Turing reduction to a problem in NP.)
It is obvious that NP ⊆ P^{NP}, but the question of whether NP ⊂ P^{NP} remains open. See polynomial hierarchy for further extensions.
The notation A^{B} also means the class of problems solvable by an algorithm in class A with an oracle for the language B. For example, P^{SAT} is the class of problems solvable in polynomial time by a deterministic Turing machine with an oracle for the Boolean satisfiability problem. When language B is complete for some class C, then A^{B}=A^{C}. In particular, since SAT is NPcomplete, P^{SAT}=P^{NP}.
Oracle machines are useful for investigating the relationship between complexity classes P and NP, by considering the relationship between P^{A} and NP^{A} for an oracle A. In particular, it has been shown that there exist languages A and B such that P^{A}=NP^{A} and P^{B}≠NP^{B} (Baker, Gill, Solovay, 1975). When a question such as this has different answers for different oracles, it is said to relativize both ways. The fact that the P=NP question relativizes both ways is taken as evidence that answering this question will be difficult, because any proof technique that relativizes (i.e., is unaffected by the addition of an oracle) will not answer the P=NP question.
It is interesting to consider the case where an oracle is chosen randomly from among all possible oracles. It has been shown that if oracle A is chosen randomly, then with probability 1, P^{A}≠NP^{A} (Bennett, Gill, 1981). When a question is true for almost all oracles, it is said to be true for a random oracle. This is sometimes taken as evidence that P≠NP; unfortunately, it is possible for a statement to be true for a random oracle, but not be true for ordinary Turing machines.
Oracles and halting problems
It is possible to posit the existence of an oracle which computes a noncomputable function, such as the answer to the halting problem or some equivalent. A machine with an oracle of this sort is a hypercomputer.
Interestingly, the halting paradox still applies to such machines; that is, although they can determine whether particular Turing machines will halt on particular inputs, they cannot determine whether machines with equivalent halting oracles will themselves halt. This fact creates a hierarchy of machines, each with a more powerful halting oracle and an even harder halting problem.
Bibliography
 Alan Turing, Systems of logic based on ordinals, Proc. London math. soc., 45, 1939
 C. Papadimitriou. Computational Complexity. AddisonWesley, 1994.
 T. P. Baker, J. Gill, R. Solovay. Relativizatons of the P =? NP Question. SIAM Journal on Computing, 4(4): 431442 (1975)
 C. H. Bennett, J. Gill. Relative to a Random Oracle A, P^{A} != NP^{A} != coNP^{A} with Probability 1. SIAM Journal on Computing, 10(1): 96113 (1981)fr:Oracle (machine de Turing)