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Exponential hierarchy : ウィキペディア英語版
Exponential hierarchy
In computational complexity theory, the exponential hierarchy is a hierarchy of complexity classes, which is an exponential time analogue of the polynomial hierarchy. As elsewhere in complexity theory, “exponential” is used in two different meanings (linear exponential bounds 2^ for a constant ''c'', and full exponential bounds 2^), leading to two versions of the exponential hierarchy:〔Sarah Mocas, Separating classes in the exponential-time hierarchy from classes in ''PH'', Theoretical Computer Science 158 (1996), no. 1–2, pp. 221–231.〕〔Anuj Dawar, Georg Gottlob, Lauri Hella, Capturing relativized complexity classes without order, Mathematical Logic Quarterly 44 (1998), no. 1, pp. 109–122.〕
*EH is the union of the classes \Sigma^E_k for all ''k'', where \Sigma^E_k=\mathrm^ for some constant ''c'' with a \Sigma^P_ oracle). One also defines \Pi^E_k=\mathrm^}. An equivalent definition is that a language ''L'' is in \Sigma^E_k if and only if it can be written in the form
::x\in L\iff\exists y_1\,\forall y_2\dots Qy_k\,R(x,y_1,\dots,y_k),
:where R(x,y_1,\dots,y_n) is a predicate computable in time 2^ (which implicitly bounds the length of ''yi''). Also equivalently, EH is the class of languages computable on an alternating Turing machine in time 2^ for some ''c'' with constantly many alternations.
*EXPH is the union of the classes \Sigma^_k, where \Sigma^_k=\mathrm^ for some constant ''c'' with a \Sigma^P_ oracle), and again \Pi^_k=\mathrm^_k=\mathrm^_k if and only if it can be written as
::x\in L\iff\exists y_1\,\forall y_2\dots Qy_k\,R(x,y_1,\dots,y_k),
:where R(x,y_1,\dots,y_k) is computable in time 2^ for some ''c'', which again implicitly bounds the length of ''yi''. Equivalently, EXPH is the class of languages computable in time 2^ on an alternating Turing machine with constantly many alternations.
We have ENE ⊆ EH ⊆ ESPACE, EXPNEXP ⊆ EXPH ⊆ EXPSPACE, and EH ⊆ EXPH.
== References ==


抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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