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Harald Hempel

Publications and source records attributed to Harald Hempel.

13 recordsLinked to original sources
arXiv2005

Algebraic Properties for Selector Functions

The nondeterministic advice complexity of the P-selective sets is known to be exactly linear. Regarding the deterministic advice complexity of the P-selective sets--i.e., the amount of Karp--Lipton advice needed for polynomial-time machines to recognize them in general--the best current upper bound is quadratic [Ko, 1983] and the best current lower bound is linear [Hemaspaandra and Torenvliet, 1996]. We prove that every associatively P-selective set is commutatively, associatively P-selective. Using this, we establish an algebraic sufficient condition for the P-selective sets to have a linear upper bound (which thus would match the existing lower bound) on their deterministic advice complexity: If all P-selective sets are associatively P-selective then the deterministic advice complexity of the P-selective sets is linear. The weakest previously known sufficient condition was P=NP. We also establish related results for algebraic properties of, and advice complexity of, the nondeterministically selective sets.

cs.CC↗
arXiv2004

All Superlinear Inverse Schemes are coNP-Hard

How hard is it to invert NP-problems? We show that all superlinearly certified inverses of NP problems are coNP-hard. To do so, we develop a novel proof technique that builds diagonalizations against certificates directly into a circuit.

cs.CC↗
arXiv2001

Using the No-Search Easy-Hard Technique for Downward Collapse

The top part of the preceding figure [figure appears in actual paper] shows some classes from the (truth-table) bounded-query and boolean hierarchies. It is well-known that if either of these hierarchies collapses at a given level, then all higher levels of that hierarchy collapse to that same level. This is a standard ``upward translation of equality'' that has been known for over a decade. The issue of whether these hierarchies can translate equality {\em downwards\/} has proven vastly more challenging. In particular, with regard to the figure above, consider the following claim: $$P_{m-tt}^{Σ_k^p} = P_{m+1-tt}^{Σ_k^p} \implies DIFF_m(Σ_k^p) coDIFF_m(Σ_k^p) = BH(Σ_k^p). (*) $$ This claim, if true, says that equality translates downwards between levels of the bounded-query hierarchy and the boolean hierarchy levels that (before the fact) are immediately below them. Until recently, it was not known whether (*) {\em ever\/} held, except for the degenerate cases $m=0$ and $k=0$. Then Hemaspaandra, Hemaspaandra, and Hempel \cite{hem-hem-hem:j:downward-translation} proved that (*) holds for all $m$, for $k > 2$. Buhrman and Fortnow~\cite{buh-for:j:two-queries} then showed that, when $k=2$, (*) holds for the case $m = 1$. In this paper, we prove that for the case $k=2$, (*) holds for all values of $m$. Since there is an oracle relative to which ``for $k=1$, (*) holds for all $m$'' fails \cite{buh-for:j:two-queries}, our achievement of the $k=2$ case cannot to be strengthened to $k=1$ by any relativizable proof technique. The new downward translation we obtain also tightens the collapse in the polynomial hierarchy implied by a collapse in the bounded-query hierarchy of the second level of the polynomial hierarchy.

cs.CC↗
arXiv1999

Query Order and the Polynomial Hierarchy

Hemaspaandra, Hempel, and Wechsung [cs.CC/9909020] initiated the field of query order, which studies the ways in which computational power is affected by the order in which information sources are accessed. The present paper studies, for the first time, query order as it applies to the levels of the polynomial hierarchy. We prove that the levels of the polynomial hierarchy are order-oblivious. Yet, we also show that these ordered query classes form new levels in the polynomial hierarchy unless the polynomial hierarchy collapses. We prove that all leaf language classes - and thus essentially all standard complexity classes - inherit all order-obliviousness results that hold for P.

cs.CC↗
arXiv1999

Translating Equality Downwards

Downward translation of equality refers to cases where a collapse of some pair of complexity classes would induce a collapse of some other pair of complexity classes that (a priori) one expects are smaller. Recently, the first downward translation of equality was obtained that applied to the polynomial hierarchy-in particular, to bounded access to its levels [cs.CC/9910007]. In this paper, we provide a much broader downward translation that extends not only that downward translation but also that translation's elegant enhancement by Buhrman and Fortnow. Our work also sheds light on previous research on the structure of refined polynomial hierarchies, and strengthens the connection between the collapse of bounded query hierarchies and the collapse of the polynomial hierarchy.

cs.CC↗
arXiv1999

What's Up with Downward Collapse: Using the Easy-Hard Technique to Link Boolean and Polynomial Hierarchy Collapses

During the past decade, nine papers have obtained increasingly strong consequences from the assumption that boolean or bounded-query hierarchies collapse. The final four papers of this nine-paper progression actually achieve downward collapse---that is, they show that high-level collapses induce collapses at (what beforehand were thought to be) lower complexity levels. For example, for each $k\geq 2$ it is now known that if $\psigkone=\psigktwo$ then $\ph=\sigmak$. This article surveys the history, the results, and the technique---the so-called easy-hard method---of these nine papers.

cs.CC↗
arXiv1999

R_{1-tt}^{SN}(NP) Distinguishes Robust Many-One and Turing Completeness

Do complexity classes have many-one complete sets if and only if they have Turing-complete sets? We prove that there is a relativized world in which a relatively natural complexity class-namely a downward closure of NP, \rsnnp - has Turing-complete sets but has no many-one complete sets. In fact, we show that in the same relativized world this class has 2-truth-table complete sets but lacks 1-truth-table complete sets. As part of the groundwork for our result, we prove that \rsnnp has many equivalent forms having to do with ordered and parallel access to $\np$ and $\npinterconp$.

cs.CC↗
arXiv1999

An Introduction to Query Order

Hemaspaandra, Hempel, and Wechsung [cs.CC/9909020] raised the following questions: If one is allowed one question to each of two different information sources, does the order in which one asks the questions affect the class of problems that one can solve with the given access? If so, which order yields the greater computational power? The answers to these questions have been learned-inasfar as they can be learned without resolving whether or not the polynomial hierarchy collapses-for both the polynomial hierarchy and the boolean hierarchy. In the polynomial hierarchy, query order never matters. In the boolean hierarchy, query order sometimes does not matter and, unless the polynomial hierarchy collapses, sometimes does matter. Furthermore, the study of query order has yielded dividends in seemingly unrelated areas, such as bottleneck computations and downward translation of equality. In this article, we present some of the central results on query order. The article is written in such a way as to encourage the reader to try his or her own hand at proving some of these results. We also give literature pointers to the quickly growing set of related results and applications.

cs.CC↗
arXiv1999

Self-Specifying Machines

We study the computational power of machines that specify their own acceptance types, and show that they accept exactly the languages that $\manyonesharp$-reduce to NP sets. A natural variant accepts exactly the languages that $\manyonesharp$-reduce to P sets. We show that these two classes coincide if and only if $\psone = \psnnoplusbigohone$, where the latter class denotes the sets acceptable via at most one question to $\sharpp$ followed by at most a constant number of questions to $\np$.

cs.CC↗
arXiv1999

A Downward Collapse within the Polynomial Hierarchy

Downward collapse (a.k.a. upward separation) refers to cases where the equality of two larger classes implies the equality of two smaller classes. We provide an unqualified downward collapse result completely within the polynomial hierarchy. In particular, we prove that, for k > 2, if $\psigkone = \psigktwo$ then $\sigmak = \pik = \ph$. We extend this to obtain a more general downward collapse result.

cs.CC↗
arXiv1999

Query Order

We study the effect of query order on computational power, and show that $\pjk$-the languages computable via a polynomial-time machine given one query to the jth level of the boolean hierarchy followed by one query to the kth level of the boolean hierarchy-equals $\redttnp{j+2k-1}$ if j is even and k is odd, and equals $\redttnp{j+2k}$ otherwise. Thus, unless the polynomial hierarchy collapses, it holds that for each $1\leq j \leq k$: $\pjk = \pkj \iff (j=k) \lor (j{is even} \land k=j+1)$. We extend our analysis to apply to more general query classes.

cs.CC↗
arXiv1998

Downward Collapse from a Weaker Hypothesis

Hemaspaandra et al. proved that, for $m > 0$ and $0 < i < k - 1$: if $Σ_i^p \BoldfaceDelta DIFF_m(Σ_k^p)$ is closed under complementation, then $DIFF_m(Σ_k^p) = coDIFF_m(Σ_k^p)$. This sharply asymmetric result fails to apply to the case in which the hypothesis is weakened by allowing the $Σ_i^p$ to be replaced by any class in its difference hierarchy. We so extend the result by proving that, for $s,m > 0$ and $0 < i < k - 1$: if $DIFF_s(Σ_i^p) \BoldfaceDelta DIFF_m(Σ_k^p)$ is closed under complementation, then $DIFF_m(Σ_k^p) = coDIFF_m(Σ_k^p)$.

cs.CC↗