Weakly Consecutive Sequences

A weakly consecutive sequence (WCS) is a permutation $\sigma$ of $\{1,\dots ,k\}$ such that if an integer $d$ divides $\sigma (i)$, then $d$ also divides $\sigma (i\pm d)$ insofar as these are defined. The structure of weakly consecutive sequences is surprisingly rich, and it is difficult to find a formula for the number $N(k)$ of WCS’s of length $k$. However, for a given $k$ we describe four starting sequences, to each of which we can apply three \emph{rules} or operations to generate new WCS’s. We conjecture that any WCS can be constructed by applying these rules, which depend in an intricate way on the primality of $k$ and surrounding integers. We find bounds for $N(k)$ by analyzing these rules.