On non-repetitive sequences of arithmetic progressions:the cases $k \in \{4,5,6,7,8\}$

TL;DR

This paper proves Grytczuk's conjecture that for any integer k between 2 and 8, sequences with k+2 symbols can be constructed to avoid repetitive patterns in all d-subsequences for 1 ≤ d ≤ k, extending previous results.

Contribution

The paper introduces two new proof techniques to confirm Grytczuk's conjecture for all k in the range 2 to 8, expanding the known cases.

Findings

Confirmed Grytczuk's conjecture for k=2 to 8

Developed two different proof methods

Extended the class of k for which the conjecture holds

Abstract

A $d$-subsequence of a sequence $\varphi = x_1\dots x_n$ is a subsequence $x_i x_{i+d} x_{i+2d} \dots$, for any positive integer $d$ and any $i$, $1 \le i \le n$. A \textit{$k$-Thue sequence} is a sequence in which every $d$-subsequence, for $1 \le d \le k$, is non-repetitive, i.e. it contains no consecutive equal subsequences. In 2002, Grytczuk proposed a conjecture that for any $k$, $k+2$ symbols are enough to construct a $k$-Thue sequences of arbitrary lengths. So far, the conjecture has been confirmed for $k \in \{1,2,3,5\}$. Here, we present two different proving techniques, and confirm it for all $k$, with $2 \le k \le 8$.