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

Abstract: 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 $(s_{k,n})<em>{n=1}<sup>\infty$ is a subsequence with terms taken periodically from the Padovan-like sequence $(a</em>{k,n})<em>{n=0}<sup>\infty$ defined as: $$a</em>{k,0} = a_{k,1} = 1, a_{k, 2} = \cdots = a_{k, k} = 2$$, 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 $(s_{k,n})<em>{n=1}<sup>\infty$ discovered by Beanland et al. Furthermore, a similar result holds for the sequence $(s^{(m)}</em>{k,n})<em>{n=1}<sup>\infty$ that counts maximal Schreier sets. Finally, we prove that $$s^{(m)}</em>{k,n}\ =\ 2s_{k,n}-s_{k,n+1}, \mbox{ for all }n\ge 1.$$