The dual bkr inequality and rudich's conjecture

  • Authors:
  • Jeff Kahn;Michael Saks;Clifford Smyth

  • Affiliations:
  • Mathematics department, rutgers university, piscataway, nj, usa (e-mail: jkahn@math.rutgers.edu, saks@math.rutgers.edu);Mathematics department, rutgers university, piscataway, nj, usa (e-mail: jkahn@math.rutgers.edu, saks@math.rutgers.edu);Mathematics department, university of north carolina greensboro, greensboro, nc, usa (e-mail: cdsmyth@uncg.edu)

  • Venue:
  • Combinatorics, Probability and Computing
  • Year:
  • 2011

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Abstract

Let be a set of terms over an arbitrary (but finite) number of Boolean variables. Let U() be the set of truth assignments that satisfy exactly one term in . Motivated by questions in computational complexity, Rudich conjectured that there exist ∊, δ 0 such that, if is any set of terms for which U() contains at least a (1−∊)-fraction of all truth assignments, then there exists a term t ∈ such that at least a δ-fraction of assignments satisfy some term of sharing a variable with t [8]. We prove a stronger version: for any independent assignment of the variables (not necessarily the uniform one), if the measure of U() is at least 1 − ∊, there exists a t ∈ such that the measure of the set of assignments satisfying either t or some term incompatible with t (i.e., having no satisfying assignments in common with t) is at least $\Gd = 1-\Ge-\frac{4\Ge}{1-\Ge}$. (A key part of the proof is a correlation-like inequality on events in a finite product probability space that is in some sense dual to Reimer's inequality [11], a.k.a. the BKR inequality [5], or the van den Berg–Kesten conjecture [3].)