The theory of NP-completeness was initially built on Cook (polynomial-time Turing) reductions. Later, Karp introduced polynomial-time many-to-one reductions. A Cook reduction is more powerful than a Karp reduction since there is no restriction on the number of calls to the oracle. So, I am interested in NP-complete graph problem that does not have a known Karp reduction from a NP-complete problem.

Is there a natural graph problem known to be $NP$-complete only under Cook reduction, but not known to be NP-complete under Karp reductions?

Naturalness should disallow specific features of feasible solutions, for otherwise it is quite easy to start from well-known problem and make it a little easier by allowing specific features.

  • $\begingroup$ "Known to be NP-compete only under Cook reductions" or "only known to be NP-complete under Cook reductions"? $\endgroup$ Dec 17, 2013 at 21:05
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    $\begingroup$ NON-HAMILTONICITY. $\endgroup$ Dec 19, 2013 at 5:38
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    $\begingroup$ @MohammadAl-Turkistany HAMILTONICITY is NP-complete, so we don't expect that NON-HAMILTONICITY be NP-complete (unless NP=coNP). However, it is NP-hard with respect to Cook reductions. Now I see that this doesn't fit your description, because the problem is not in NP. $\endgroup$ Dec 19, 2013 at 16:06
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    $\begingroup$ See also this question: cstheory.stackexchange.com/questions/3333/…. $\endgroup$ Dec 19, 2013 at 16:38
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    $\begingroup$ its a known open problem, basically. they are conjectured to be different, see lutz/mayordomo ref/answer for this question many one vs turing reductions $\endgroup$
    – vzn
    Dec 19, 2013 at 17:39

1 Answer 1


I know an example of a problem that is missing two of the four features you ask for - it is not NP-complete, and it is not a problem on graphs.

Buchfuhrer and Umans (2011) show that the minimum equivalent expression problem in Boolean logic is complete for $\Sigma^P_2$ under polynomial-time Turing reductions.

Given a Boolean $(\wedge;\vee;\neg)$-formula $F$ and an integer $k$, is there an equivalent $(\wedge;\vee;\neg)$-formula of size at most $k$?

On p. 143, the authors state:

This provides a somewhat rare example of a natural problem for which a Turing reduction seems crucial (in the sense that we do not know of any simple modification or alternative methods that would give a many-one reduction).

They do not cite any other such examples from the literature. I suspect that "somewhat rare" is possibly an understatement.


David Buchfuhrer and Christopher Umans: "The complexity of Boolean formula minimization" Journal of Computer and System Sciences, Volume 77, Issue 1, January 2011, Pages 142-153


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