On k-neighbor separated permutations

TL;DR

This paper investigates the maximum size of sets of permutations where each pair differs in a specific neighbor separation pattern, providing exact and asymptotic results for various values of k.

Contribution

The authors determine the asymptotic growth of the maximum number of pairwise k-neighbor separated permutations for certain k values and propose a conjecture for even k.

Findings

For k=2^ell+1, P(n,k) = 2^{n-o(n)}.

Asymptotic limit of the growth rate as k and n increase is 2.

Exact value for even n: P(n,n)= 3n/2.

Abstract

Two permutations of $[n]=\{1,2 \ldots n\}$ are \textit{$k$-neighbor separated} if there are two elements that are neighbors in one of the permutations and that are separated by exactly $k-2$ other elements in the other permutation. Let the maximal number of pairwise $k$-neighbor separated permutations of $[n]$ be denoted by $P(n,k)$. In a previous paper, the authors have determined $P(n,3)$ for every $n$, answering a question of K\"orner, Messuti and Simonyi affirmatively. In this paper we prove that for every fixed positive integer $\ell $, $$P(n,2^\ell+1) = 2^{n-o(n)}. $$ We conjecture that for every fixed even $k$, $P(n,k)=2^{n-o(n)}$. We also show that this conjecture is asymptotically true in the following sense $$\lim_{k \rightarrow \infty} \lim_{n \rightarrow \infty} \sqrt[n]{P(n,k)}=2.$$ Finally, we show that for even $n$, $P(n,n)= 3n/2$.