Reflection on the reflection complexity

Abstract

The factor complexity ${\mathcal C}_{\mathbf u}$ of a sequence ${\mathbf u} = u_0u_1u_2 \cdots$ over a finite alphabet counts the number of factors of length $n$ occurring in $\mathbf u$, i.e., ${\mathcal C}_{\mathbf u}(n) = \#{\mathcal L}_n(\mathbf u)$, where ${\mathcal L}_n({\mathbf u)}= \{u_iu_{i+1}\cdots u_{i+n-1}: i \in \mathbb N\}$. Two factors of ${\mathcal L}_n(\mathbf u)$ are said to be equivalent if one factor is the reversal of the other one. Recently, Allouche et al. introduced the reflection complexity $r_{\mathbf u}$ which counts the number of non-equivalent factors of $\mathcal{L}_n(\mathbf u)$. They formulated the following conjecture: a sequence $\mathbf u$ is eventually periodic if and only if $r_{\mathbf u}(n+2) = r_{\mathbf u}(n)$ for some $n \in \mathbb N$. Here we prove the conjecture and characterize the sequences for which $r_{\mathbf u}(n+2) = r_{\mathbf u}(n)+1$ for every $n \in \mathbb N$ and also the sequences for which the equality is satisfied for every sufficiently large $n \in \mathbb N$.