Editing EXPSPACE

{{Short description|Set of decision problems}}
In [[computational complexity theory]], '''{{Sans-serif|EXPSPACE}}''' is the [[Set (mathematics)|set]] of all [[decision problem]]s solvable by a deterministic [[Turing machine]] in [[exponential function|exponential]] [[space complexity|space]], i.e., in <math>O(2^{p(n)})</math> space, where <math>p(n)</math> is a polynomial function of <math>n</math>. Some authors restrict <math>p(n)</math> to be a [[linear function]], but most authors instead call the resulting class {{Sans-serif|[[ESPACE]]}}. If we use a nondeterministic machine instead, we get the class {{Sans-serif|NEXPSPACE}}, which is equal to {{Sans-serif|EXPSPACE}} by [[Savitch's theorem]].

A decision problem is {{Sans-serif|EXPSPACE-complete}} if it is in {{Sans-serif|EXPSPACE}}, and every problem in {{Sans-serif|EXPSPACE}} has a [[polynomial-time many-one reduction]] to it.  In other words, there is a polynomial-time [[algorithm]] that transforms instances of one to instances of the other with the same answer.  {{Sans-serif|EXPSPACE-complete}} problems might be thought of as the hardest problems in {{Sans-serif|EXPSPACE}}.

{{Sans-serif|EXPSPACE}} is a strict superset of {{Sans-serif|[[PSPACE]]}}, {{Sans-serif|[[NP (complexity)|NP]]}}, and {{Sans-serif|[[P (complexity)|P]]}}. It contains {{Sans-serif|[[EXPTIME]]}} and is believed to strictly contain it, but this is unproven.

== Formal definition ==
In terms of {{Sans-serif|[[DSPACE]]}} and {{Sans-serif|[[NSPACE]]}},

:<math>\mathsf{EXPSPACE} = \bigcup_{k\in\mathbb{N}} \mathsf{DSPACE}\left(2^{n^k}\right) = \bigcup_{k\in\mathbb{N}} \mathsf{NSPACE}\left(2^{n^k}\right)</math>

== Examples of problems ==

=== Formal languages ===
An example of an {{Sans-serif|EXPSPACE-complete}} problem is the problem of recognizing whether two [[regular expression]]s represent different languages, where the expressions are limited to four operators: union, [[concatenation]], the [[Kleene star]] (zero or more copies of an expression), and squaring (two copies of an expression).<ref>Meyer, A.R. and [[Larry Stockmeyer|L. Stockmeyer]]. [https://people.csail.mit.edu/meyer/rsq.pdf The equivalence problem for regular expressions with squaring requires exponential space]. ''13th IEEE Symposium on Switching and Automata Theory'', Oct 1972, pp.125&ndash;129.</ref>

=== Logic ===
Alur and Henzinger extended [[linear temporal logic]] with times (integer) and prove that the validity problem of their logic is EXPSPACE-complete.<ref>{{Cite journal|last1=Alur|first1=Rajeev|last2=Henzinger|first2=Thomas A.|date=1994-01-01|title=A Really Temporal Logic|journal=J. ACM|volume=41|issue=1|pages=181–203|doi=10.1145/174644.174651|issn=0004-5411|doi-access=free}}</ref>

Reasoning in the first-order theory of the real numbers with +, ×, = is in EXPSPACE and was conjectured to be EXPSPACE-complete in 1986.<ref>{{Cite journal |last=Ben-Or |first=Michael |last2=Kozen |first2=Dexter |last3=Reif |first3=John |date=1986-04-01 |title=The complexity of elementary algebra and geometry |url=https://www.sciencedirect.com/science/article/pii/0022000086900292 |journal=Journal of Computer and System Sciences |volume=32 |issue=2 |pages=251–264 |doi=10.1016/0022-0000(86)90029-2 |issn=0022-0000}}</ref>

=== Petri nets ===
The coverability problem for [[Petri Nets]] is {{Sans-serif|EXPSPACE}}-complete.<ref>{{cite journal | 
  author = Charles Rackoff |
  title = The covering and boundedness problems for vector addition systems |
  journal = Theoretical Computer Science |
  pages = 223–231 |
  date = 1978}}</ref>

The [[reachability problem]] for Petri nets was known to be {{Sans-serif|EXPSPACE}}-hard for a long time,<ref>{{cite journal | last = Lipton | first = R. | url = http://citeseer.ist.psu.edu/contextsummary/115623/0 | title = The Reachability Problem Requires Exponential Space | journal = Technical Report 62 | publisher = Yale University | date = 1976 }}</ref> but shown to be [[Nonelementary problem|nonelementary]],<ref>{{cite conference | author = Wojciech Czerwiński Sławomir Lasota Ranko S Lazić Jérôme Leroux Filip Mazowiecki
| title = The reachability problem for Petri nets is not elementary | book-title = STOC 19 | date = 2019}}</ref> so probably not in {{Sans-serif|EXPSPACE}}. In 2022 it was shown to be [[Ackermann function|Ackermann]]-complete.<ref name=":1">{{Cite book |last=Leroux |first=Jerome |chapter=The Reachability Problem for Petri Nets is Not Primitive Recursive |date=February 2022 |title=2021 IEEE 62nd Annual Symposium on Foundations of Computer Science (FOCS) |chapter-url=https://ieeexplore.ieee.org/document/9719763 |publisher=IEEE |pages=1241–1252 |doi=10.1109/FOCS52979.2021.00121 |isbn=978-1-6654-2055-6|arxiv=2104.12695 }}</ref><ref name=":0">{{Cite web |last=Brubaker |first=Ben |date=4 December 2023 |title=An Easy-Sounding Problem Yields Numbers Too Big for Our Universe |url=https://www.quantamagazine.org/an-easy-sounding-problem-yields-numbers-too-big-for-our-universe-20231204/ |website=[[Quanta Magazine]]}}</ref>

==See also==
*[[Game complexity]]

== References ==

<references />
*{{cite journal |last1=Berman |first1=Leonard |title=The complexity of logical theories |journal=Theoretical Computer Science |date=1 May 1980 |volume=11 |issue=1 |pages=71–77 |doi=10.1016/0304-3975(80)90037-7|doi-access=free }}
* {{cite book | author = Michael Sipser | year = 1997 | title = Introduction to the Theory of Computation | publisher = PWS Publishing | isbn = 0-534-94728-X | url-access = registration | url = https://archive.org/details/introductiontoth00sips | author-link = Michael Sipser }} Section 9.1.1: Exponential space completeness, pp.&nbsp;313&ndash;317. Demonstrates that determining equivalence of regular expressions with exponentiation is EXPSPACE-complete.

{{ComplexityClasses}}

[[Category:Complexity classes]]