Affiliation:
1. University of Chicago, Departments of Mathematics and Computer Science, Chicago IL, USA
Abstract
We exhibit an unusually strong tradeoff in propositional proof complexity that significantly deviates from the established pattern of almost all results of this kind. Namely, restrictions on one resource (width, in our case) imply an increase in another resource (tree-like size) that is exponential not only with respect to the complexity of the original problem, but also to the whole class of
all
problems of the same bit size. More specifically, we show that for any parameter
k
=
k
(
n
), there are unsatisfiable
k
-CNFs that possess refutations of width
O
(
k
), but such that any tree-like refutation of width
n
1 − ϵ
/
k
must necessarily have
doubly
exponential size exp (
n
Ω(
k
)
). This means that there exist contradictions that allow narrow refutations, but in order to keep the size of such a refutation even within a single exponent, it must necessarily use a high degree of parallelism.
Our construction and proof methods combine, in a non-trivial way, two previously known techniques: the hardness escalation method based on substitution formulas and expansion. This combination results in a
hardness compression
approach that strives to preserve hardness of a contradiction while significantly decreasing the number of its variables.
Funder
Russian Science Foundation
Publisher
Association for Computing Machinery (ACM)
Subject
Artificial Intelligence,Hardware and Architecture,Information Systems,Control and Systems Engineering,Software
Cited by
14 articles.
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