Almost isoperimetric subsets of the discrete cube

  • Authors:
  • David Ellis

  • Affiliations:
  • St john's college, cambridge, uk (e-mail: dce27@cam.ac.uk)

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

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Abstract

We show that a set A ⊂ {0, 1}n with edge-boundary of size at most \[|A| (\log_{2}(2^{n}/|A|) + \epsilon)\] can be made into a subcube by at most (2ε/log2(1/ε))|A| additions and deletions, provided ε is less than an absolute constant. We deduce that if A ⊂ {0, 1}n has size 2t for some t ∈ ℕ, and cannot be made into a subcube by fewer than δ|A| additions and deletions, then its edge-boundary has size at least \[|A| \log_{2}(2^{n}/|A|) + |A| \delta \log_{2}(1/\delta) = 2^{t}(n-t+\delta \log_{2}(1/\delta)),\] provided δ is less than an absolute constant. This is sharp whenever δ = 1/2j for some j ∈ {1, 2,. . ., t}.