Exponential hierarchy explained

In computational complexity theory, the exponential hierarchy is a hierarchy of complexity classes that 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 ^{cn} for a constant c, and full exponential bounds 2 ^{n ^{c}} ), leading to two versions of the exponential hierarchy.^{[1] [2]} This hierarchy is sometimes also referred to as the weak exponential hierarchy, to differentiate it from the strong exponential hierarchy.^[3]

EH

The complexity class EH is the union of the classes

\Sigma ^{E} _{k} for all k, where \Sigma ^{\Sigma ^{P} _{k -1}} _{k = NE} (i.e., languages computable in nondeterministic time 2 ^{cn} for some constant c with a \Sigma ^{P} _{k -1} oracle) and \Sigma ^{E} _{0} = E . One also defines \Pi ^{\Sigma ^{P} _{k -1}} _{k = coNE} and \Delta ^{\Sigma ^{P} _{k -1}} _{k = E} .

An equivalent definition is that a language L is in

\Sigma ^{E} _{k} if and only if it can be written in the formx\in L\iff\exists y _{1\forall} y _{2...} Qy _{k} R(x,y _{1,\ldots,y} _{k),}

where

R(x,y _{1,\ldots,y} _{n)} is a predicate computable in time 2 ^{c|x|} (which implicitly bounds the length of y_i). Also equivalently, EH is the class of languages computable on an alternating Turing machine in time 2 ^{cn} for some c with constantly many alternations.

EXPH

EXPH is the union of the classes

\Sigma ^{EXP} _{k} , where \Sigma ^{\Sigma ^{P} _{k -1}} _{k = NEXP} (languages computable in nondeterministic time 2 ^{n ^{c}} for some constant c with a \Sigma ^{P} _{k -1} oracle), \Sigma ^{EXP} _{0} = EXP , and again:\Pi ^{\Sigma ^{P} _{k -1}} _{k = coNEXP} , \Delta ^{\Sigma ^{P} _{k -1}} _{k = EXP} .

A language L is in

\Sigma ^{EXP} _{k} if and only if it can be written asx\in L\iff\exists y _{1} \forall y _{2} ... Qy _{k} R(x,y _{1,\ldots,y} _{k),}

where

R(x,y _{1,\ldots,y} _{k)} is computable in time 2 ^{|x| ^{c}} for some c, which again implicitly bounds the length of y_i. Equivalently, EXPH is the class of languages computable in time 2 ^{n ^{c}} on an alternating Turing machine with constantly many alternations.

Comparison

E ⊆ NE ⊆ EH⊆ ESPACE,

EXP ⊆ NEXP ⊆ EXPH⊆ EXPSPACE,

EH ⊆ EXPH.

Notes and References

1. Sarah Mocas, Separating classes in the exponential-time hierarchy from classes in PH, Theoretical Computer Science 158 (1996), no. 1–2, pp. 221–231.
2. Anuj Dawar, Georg Gottlob, Lauri Hella, Capturing relativized complexity classes without order, Mathematical Logic Quarterly 44 (1998), no. 1, pp. 109–122.
3. Hemachandra. Lane A.. 1989. The strong exponential hierarchy collapses. Journal of Computer and System Sciences. en. 39. 3. 299–322.