On a Relation between Schreier-type Sets and a Modification of Turán Graphs

Abstract: Recently, a relation between Schreier-type sets and Tur\'{a}n graphs was discovered. In this note, we give a combinatorial proof and obtain a generalization of the relation. Specifically, for $p, q\ge 1$, let $$\mathcal{A}_q := {F\subset\mathbb{N}: |F| = 1 \mbox{ or }F\mbox{ is an arithmetic progression with difference } q}$$ and $$Sr(n, p, q)\ :=\ #{F\subset {1, \ldots, n}\,:\, p\min F\ge |F|\mbox{ and }F\in \mathcal{A}_q}.$$ We show that $Sr(n, p, q) \ =\ T(n+1, pq+1, q),$ where $T(\cdot, \cdot, \cdot)$ is the number of edges of an $n$-vertex graph that is a modification of Tur\'{a}n graphs. We also prove that $Sr(n,p,q)$ is the partial sum of certain sequences.