English

Permutations with few inversions are locally uniform

Combinatorics 2022-10-21 v2

Abstract

We prove that permutations with few inversions exhibit a local-global dichotomy in the following sense. Suppose σ{\boldsymbol\sigma} is a permutation chosen uniformly at random from the set of all permutations of [n][n] with exactly m=m(n)n2m=m(n)\ll n^2 inversions. If i<ji<j are chosen uniformly at random from [n][n], then σ(i)<σ(j){\boldsymbol\sigma}(i)<{\boldsymbol\sigma}(j) asymptotically almost surely. However, if ii and jj are chosen so that jim/nj-i\ll m/n, and mn2/log2nm \ll n^2/\log^2 n, then limnP[σ(i)<σ(j)]=12\lim_{n\to\infty}\mathbb{P}\big[{\boldsymbol\sigma}(i)<{\boldsymbol\sigma}(j)\big]=\frac{1}{2}. Moreover, if k=k(n)m/nk=k(n)\ll \sqrt{m/n}, then the restriction of σ{\boldsymbol\sigma} to a random kk-point interval is asymptotically uniformly distributed over Sk\mathcal{S}_k. Thus, knowledge of the local structure of σ{\boldsymbol\sigma} reveals nothing about its global form. We establish that m/n\sqrt{m/n} is the threshold for local uniformity and m/nm/n the threshold for inversions, and determine the behaviour in the critical windows. As pointed out by a referee, there are flaws in the proofs that do not seem easily rectifiable (see comments on pages 9 and 15). So the results stated above have not been established.

Keywords

Cite

@article{arxiv.1908.07277,
  title  = {Permutations with few inversions are locally uniform},
  author = {David Bevan},
  journal= {arXiv preprint arXiv:1908.07277},
  year   = {2022}
}

Comments

17 pages; there are flaws in the proofs, so the claimed results have not been established

R2 v1 2026-06-23T10:51:59.780Z