EXPTIME

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In computational complexity theory, the complexity class EXPTIME (sometimes called EXP) is the set of all decision problems solvable by a deterministic Turing machine in O(2^p(n)) time, where p(n) is a polynomial function of n.

In terms of DTIME,

$\mbox{EXPTIME} = \bigcup_{k \in \mathbb{N} } \mbox{ DTIME } \left( 2^{ n^k } \right) .$

We know

P $\subseteq$ NP $\subseteq$ PSPACE $\subseteq$ EXPTIME $\subseteq$ NEXPTIME $\subseteq$ EXPSPACE

and also, by the time hierarchy theorem and the space hierarchy theorem, that

P $\subsetneq$ EXPTIME and PSPACE $\subsetneq$ EXPSPACE

so at least one of the first three inclusions and at least one of the last three inclusions must be proper, but it is not known which ones are. Most experts believe all the inclusions are proper. It's also known that if P = NP, then EXPTIME = NEXPTIME, the class of problems solvable in exponential time by a nondeterministic Turing machine.¹

EXPTIME can also be reformulated as the space class APSPACE, the problems that can be solved by an alternating Turing machine in polynomial space. This is one way to see that PSPACE $\subseteq$ EXPTIME, since an alternating Turing machine is at least as powerful as a deterministic Turing machine.²

1 EXPTIME-complete
2 See also
3 External links
4 References
5 Footnotes

[edit] EXPTIME-complete

A decision problem is EXPTIME-complete if it is in EXPTIME, and every problem in EXPTIME 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. Problems that are EXPTIME-complete might be thought of as the hardest problems in EXPTIME. Notice that although we don't know if NP is a subset of P or not, we do know that EXPTIME-complete problems are not in P; it has been proven that these problems cannot be solved in polynomial time.

In computability theory, one of the basic undecidable problems is that of deciding whether a deterministic Turing machine (DTM) accepts a particular input. One of the most fundamental EXPTIME-complete problems is a simpler version of this which asks if a DTM accepts an input in at most k steps. It is in EXPTIME because a trivial simulation requires O(k) time, and the input k is encoded using O(log k) bits.⁴ It is EXPTIME-complete because, roughly speaking, we can use it to determine if a machine solving an EXPTIME problem accepts in an exponential number of steps; it will not use more.

Other examples of EXPTIME-complete problems include the problem of evaluating a position in generalized Chess, Checkers, or Go (with Japanese ko rules). These games have a chance of being EXPTIME-complete because games can last for a number of moves that is exponential in the size of the board. In the Go example, the non-repetition rule is sufficiently intractable to imply EXPTIME-completeness, but it is not known if Go with (more tractable) American or Chinese rules is EXPTIME-complete.

By contrast, generalized games that can last for a number of moves that is polynomial in the size of the board are often PSPACE-complete. The same is true of exponentially long games in which non-repetition is automatic.

Another set of important EXPTIME-complete problems relates to succinct circuits. Succinct circuits are simple machines used to describe graphs in exponentially less space. They accept two vertex numbers as input and output whether there is an edge between them. If solving a problem on a graph in a natural representation, such as an adjacency matrix, is P-complete, then solving the same problem on a succinct circuit representation is EXPTIME-complete, because the input is exponentially smaller.³

[edit] See also

[edit] External links

[edit] References

C. Papadimitriou. Computational Complexity. Addison-Wesley, 1994. ISBN 0-201-53082-1. Section 20.1: Exponential time, pp. 491–499.

Chris Umans. CS 21: Lecture 13 notes.

[edit] Footnotes

1. Papadimitriou, section 20.1, pg.491.

2. Papadimitriou, section 20.1, Corollary 3, pg.495.

3. Papadimitriou, section 20.1, pg.492.

4. Umans, slide 24.

Important complexity classes (more)
P • NP • co-NP • NP-C • co-NP-C • NP-hard • UP • #P • #P-C • L • NL • NC • P-C • PSPACE • PSPACE-C • EXPTIME • EXPSPACE • PR • RE • Co-RE • RE-C • Co-RE-C • R • BQP • BPP • RP • ZPP • PCP • IP • PH