## On polynomial-time truth-table reduciblity of intractable sets to p- selective sets.(English)Zbl 0722.68059

Summary: The existence of sets not being $$\leq^{P}_{tt}$$-reducible to low sets is investigated for several complexity classes such as UP, NP, the polynomial-time hierarchy, PSPACE, and EXPTIME. The p-selective sets are mainly considered as a class of low sets. Such investigations were done in many earlier works, but almost all of these have dealt with positive reductions in order to imply the strongest consequence such as $$P=NP$$ under the assumption that all sets in NP are polynomial-time reducible to low sets. Currently, there seems to be some difficulty in obtaining the same strong results under nonpositive reducibilities. The purpose of this paper is to develop a useful technique to show for many complexity classes that if each set in the class is polynomial-time reducible to a p-selective set via a nonpositive reduction, then the class is already contained in P. The following results are shown:
(1) If each set in UP is $$\leq^{P}_{tt}$$-reducible to a p-selective set, then $$P=UP.$$
(2) If each set in NP is $$\leq^{P}_{tt}$$-reducible to a p-selective set, then $$P=FewP$$ and $$R=NP.$$
(3) If each set in $$\Delta^{P}_{2}$$ is $$\leq^{P}_{tt}$$-reducible to a p-selective set, then $$P=NP.$$
(4) If each set in PSPACE is $$\leq^{P}_{tt}$$-reducible to a p- selective set, then $$P=PSPACE.$$
(5) There is a set in EXPTIME that is not $$\leq^{P}_{tt}$$-reducible to any p-selective set.
It remains open whether $$P=NP$$ follows from a weaker assumption that each set in NP is $$\leq^{P}_{tt}$$-reducible to a p-selective set.

### MSC:

 68Q15 Complexity classes (hierarchies, relations among complexity classes, etc.)
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### References:

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