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The polynomial-time hierarchy and sparse oracles

Published: 01 May 1986 Publication History

Abstract

Questions about the polynomial-time hierarchy are studied. In particular, the questions, “Does the polynomial-time hierarchy collapse?” and “Is the union of the hierarchy equal to PSPACE?” are considered, along with others comparing the union of the hierarchy with certain probabilistic classes. In each case it is shown that the answer is “yes” if and only if for every sparse set S, the answer is “yes” when the classes are relativized to S if and only if there exists a sparse set S such that the answer is “yes” when the classes are relativized to S. Thus, in each case the question is answered if it is answered for any arbitrary sparse oracle set.
Long and Selman first proved that the polynomial-time hierarchy collapses if and only if for every sparse set S, the hierarchy relative to S collapses. This result is re-proved here by a different technique.

References

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Graham Ernest Farr

Concerning the question of whether or not the Polynomial Hierarchy (PH) of Meyer and Stockmeyer is indeed an infinite hierarchy of distinct classes, the authors prove: (1)If k ?9T2, then the Polynomial Hierarchy collapses to &Sgr; p-k- if and only if for every sparse set S the polynomial hierarchy relative to S collapses to &Sgr; p-k-( S). (2)The Polynomial Hierarchy is a proper infinite hierarchy if and only if for every sparse set S the Polynomial Hierarchy relative to S is a proper infinite hierarchy. Result (1) was proved first, by a related technique, in a paper by Long and Selman [1] that appears in the same issue and can be regarded as a companion paper to the one under review. If the oracle set does not have to be sparse, then there are relativizations of PH that are infinite [2] and other relativizations of PH that collapse. At present such results do not appear to tell us much about the unrelativized PH. If the oracle is sparse, however, results (1) and (2) directly relate to the collapsing or otherwise of the relativized and unrelativized Polynomial Hierarchies. Thus, it would suffice to answer our opening question relative to any sparse oracle set. Similar results are found, by the same techniques, for the questions of whether PH equals PSPACE, whether Valiant's counting class D#P is contained in PH, and whether Gill's probabilistic class PP is contained in PH. The first two of these questions were also studied in the same vein in [1]. Long and Selman also obtain results analogous to result (1) for relativizations of the P=__ __NP and NP=__ __ co-NP questions with tally sets as oracles. The main results of this paper and [1] are the first of their kind, although it is worth noting another current line of research in which the model of relativized computation (rather than the oracle set) is restricted. Some results similar in flavor to those discussed here have been obtained. (See, for example, Book's recent survey [3].) The paper under review is an interesting and worthwhile contribution to the theory of relativized complexity classes. It contains a full discussion of the results and of related work by others.

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Published In

cover image Journal of the ACM
Journal of the ACM  Volume 33, Issue 3
July 1986
219 pages
ISSN:0004-5411
EISSN:1557-735X
DOI:10.1145/5925
Issue’s Table of Contents

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Association for Computing Machinery

New York, NY, United States

Publication History

Published: 01 May 1986
Published in JACM Volume 33, Issue 3

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