Schreier Sets of Multiples of an Integer, Linear Recurrence, and Pascal Triangle

Hung Viet Chu, Zachary Louis Vasseur

Published 2025-06-17, updated 2025-06-20Version 2

A finite nonempty set $F$ is said to be Schreier (maximal Schreier, respectively) if $\min F\ge |F|$ ($\min F = |F|$, respectively). For $k,n\in\mathbb{N}$, let $$s_{k,n}\ :=\ |\{F\subset\{k, 2k,\ldots, nk\}\,:\, F\mbox{ is Schreier and }nk\in F\}|.$$ We show that $(s_{k,n})_{n=1}^\infty$ is a subsequence with terms taken periodically from the Padovan-like sequence $(a_{k,n})_{n=0}^\infty$ defined as: $a_{k,0} = a_{k,1} = 1, a_{k, 2} = \cdots = a_{k, k} = 2$, and $$a_{k, n}\ =\ a_{k,n-k} + a_{k,n-k-1},\mbox{ for } n\ge k+1.$$ As an application, we obtain an alternative proof of the linear recurrence of $(s_{k,n})_{n=1}^\infty$ discovered by Beanland et al. Furthermore, a similar result holds for the sequence $(s^{(m)}_{k,n})_{n=1}^\infty$ that counts maximal Schreier sets. Finally, we prove that $$s^{(m)}_{k,n}\ =\ 2s_{k,n}-s_{k,n+1}, \mbox{ for all }n\ge 1.$$