Daniel Rubery | Searcharxiv

arXiv2016

Closure and Nonclosure Properties of the Compressible and Rankable Sets

The rankable and compressible sets have been studied for more than a quarter of a century, ever since Allender [1] and Goldberg and Sipser [6] introduced the formal study of polynomial-time ranking. Yet even after all that time, whether the rankable and compressible sets are closed under the most important boolean and other operations remains essentially unexplored. The present paper studies these questions for both polynomial-time and recursion-theoretic compression and ranking, and for almost every case arrives at a Closed, a Not-Closed, or a Closed-Iff-Well-Known-Complexity-Classes-Collapse result for the given operation. Even though compression and ranking classes are capturing something quite natural about the structure of sets, it turns out that they are quite fragile with respect to closure properties, and many fail to possess even the most basic of closure properties. For example, we show that with respect to the join (aka disjoint union) operation: the P-rankable sets are not closed, whether the semistrongly P-rankable sets are closed is closely linked to whether P = UP $\cap$ coUP, and the strongly P-rankable sets are closed.

cs.LO↗

arXiv2016

Recursion-Theoretic Ranking and Compression

For which sets A does there exist a mapping, computed by a total or partial recursive function, such that the mapping, when its domain is restricted to A, is a 1-to-1, onto mapping to $\Sigma^*$? And for which sets A does there exist such a mapping that respects the lexicographical ordering within A? Both cases are types of perfect, minimal hash functions. The complexity-theoretic versions of these notions are known as compression functions and ranking functions. The present paper defines and studies the recursion-theoretic versions of compression and ranking functions, and in particular studies the question of which sets have, or lack, such functions. Thus, this is a case where, in contrast to the usual direction of notion transferal, notions from complexity theory are inspiring notions, and an investigation, in computability theory. We show that the rankable and compressible sets broadly populate the 1-truth-table degrees, and we prove that every nonempty coRE cylinder is recursively compressible.

cs.LO↗