A self-avoiding curve associated with sums of digits

Abstract

For each $n\in N ^{\ast }$, we write $s_{n}=\left( 1,\ldots ,1,0\right) $ with $n$ times $1$. For each $a \in N$, we consider the binary representation $\left( a_{i}\right) _{i\in -N }$ of $a$ with $a_{i}=0$ for nearly each $i$; we denote by $\alpha _{n}(a)$ the number of integers $i$ such that $\left( a_{i}, \ldots ,a_{i+n} \right) =s_{n}$. We consider the curve $C_{n}=\left( S_{n,k}\right) _{k\in N ^{\ast }}$ which consists of consecutive segments of length $1$ such that, for each $k$, $S_{n,k+1}$ is obtained from $S_{n,k}$ by turning right if $k+\alpha _{n}(k)-\alpha _{n}(k-1)$ is even and left otherwise. $C_{1}$ is self-avoiding since it is the curve associated to the alternating folding sequence. In [1], M. Mend\`es France and J. Shallit conjectured that the curves $C_{n}$ for $n\geq 2$ are also self-avoiding. In the present paper, we show that this property is true for $n=2$. We also prove that $C_{2}$ has some properties similar to those which were shown in [2], [3] and [4] for folding curves.