Avoiding Monotone Arithmetic Progressions in Permutations of Integers
A permutation of the integers avoiding monotone arithmetic progressions of length $6$ was constructed in (Geneson, 2018). We construct a permutation of the integers avoiding monotone arithmetic progressions of length $5$. We also construct permutations such that $\beta_{\mathbb{Z^+}}(4)=1$, $\beta_{\mathbb{Z}}(3)\geq\frac{3}{10}$, $\beta_{\mathbb{Z}}(4)\geq\frac{2}{3}$ and $\alpha_{\mathbb{Z}}(4)=1$. In (Davis et al, 1977) they constructed a doubly infinite permutation of the positive integers that avoids monotone arithmetic progressions of length $4$. We construct a doubly infinite permutation of the integers avoiding monotone arithmetic progressions of length $5$. A permutation of the positive integers that avoided monotone arithmetic progressions of length $4$ with odd common difference was constructed in (LeSaulnier and Vijay, 2011). We generalise this result and show that for each $k\geq 1$, there exists a permutation of the integers that avoids monotone arithmetic progressions of length $4$ with common difference not divisible by $2^k$. In addition, we also specify the structure of permutations of $[1,n]$ that avoid length $3$ monotone arithmetic progressions mod $n$ as defined in (Davis et al, 1977) and provide a multiplicative result on permutations that avoid length $k$ monotone arithmetic progressions mod $n$.