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

Abstract

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.