PH was first defined by Larry Stockmeyer. It is a special case of hierarchy of bounded alternating Turing machine. It is contained in P#P = PPP (by Toda's theorem; the class of problems that are decidable by a polynomial time Turing machine with access to a #P or equivalently PP oracle), and also in PSPACE.
PH contains almost all well-known complexity classes inside PSPACE; in particular, it contains P, NP, and co-NP. It even contains probabilistic classes such as BPP and RP. However, there is some evidence that BQP, the class of problems solvable in polynomial time by a quantum computer, is not contained in PH (Aaronson 2010).
P = NP if and only if P = PH. This may simplify a potential proof of P ≠ NP, since it's only necessary to separate P from the more general class PH.
- Larry J. Stockmeyer, "The polynomial hierarchy", Theoretical Computer Science, Vol. 3 (1976), pp. 1–22.
- Scott Aaronson, BQP and the Polynomial Hierarchy, ACM STOC (2010), arXiv:0910.4698, ECCC TR09-104.
- Complexity Zoo: PH
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