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

Recently, a relation between Schreier-type sets and Turán 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 Turán graphs. We also prove that $Sr(n,p,q)$ is the partial sum of certain sequences.

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