An Erdős--Szekeres type result for words with repeats

Abstract

We prove an Erdős--Szekeres type result for finite words over $\mathbb{N}$ with repeated values. Specifically, we define a \emph{repeat} in a word to be an occurrence of a value which is not its first occurrence. We define an occurrence of a \emph{pattern} $\pi$ in a word $w$ to be a (not necessarily consecutive) subword of $w$ that is order isomorphic to $\pi$. In this note, we show that every word with $k{n}^6+1$ repeats contains one of the following patterns: $0^{k+2}$, $0011\cdots nn$, $nn\cdots 1100$, $012\cdots n012\cdots n$, $012\cdots nn\cdots 210$, $n\cdots 210012\cdots n$, $n\cdots 210n\cdots 210$. Moreover, when $k=1$, we show that this is best possible by constructing a word with $n^6$ repeats that does not contain any of these patterns.

11 pages, 9 figures