English

Almost isoperimetric subsets of the discrete cube

Combinatorics 2013-11-28 v2

Abstract

We show that a set A{0,1}nA \subset \{0,1\}^{n} with edge-boundary of size at most A(log2(2n/A)+ϵ)|A| (\log_{2}(2^{n}/|A|) + \epsilon) can be made into a subcube by at most (2ϵ/log2(1/ϵ))A(2 \epsilon/\log_{2}(1/\epsilon))|A| additions and deletions, provided ϵ\epsilon is less than an absolute constant. We deduce that if A{0,1}nA \subset \{0,1\}^{n} has size 2t2^{t} for some tNt \in \mathbb{N}, and cannot be made into a subcube by fewer than δA\delta |A| additions and deletions, then its edge-boundary has size at least Alog2(2n/A)+Aδlog2(1/δ)=2t(nt+δlog2(1/δ))|A| \log_{2}(2^{n}/|A|) + |A| \delta \log_{2}(1/\delta) = 2^{t}(n-t+\delta \log_{2}(1/\delta)), provided δ\delta is less than an absolute constant. This is sharp whenever δ=1/2j\delta = 1/2^{j} for some j{1,2,,t}j \in \{1,2,\ldots,t\}.

Keywords

Cite

@article{arxiv.1310.8179,
  title  = {Almost isoperimetric subsets of the discrete cube},
  author = {David Ellis},
  journal= {arXiv preprint arXiv:1310.8179},
  year   = {2013}
}

Comments

Typos corrected in Conjectures 12 and 13

R2 v1 2026-06-22T01:57:29.861Z