Arithmetic Progressions in the Graphs of Slightly Curved Sequences (1807.06971v3)

Abstract: A strictly increasing sequence of positive integers is called a slightly curved sequence with small error if the sequence can be well-approximated by a function whose second derivative goes to zero faster than or equal to $1/x^\alpha$ for some $\alpha>0$. In this paper, we prove that arbitrarily long arithmetic progressions are contained in the graph of a slightly curved sequence with small error. Furthermore, we extend Szemer\'edi's theorem to a theorem about slightly curved sequences. As a corollary, it follows that the graph of the sequence ${\lfloor{n^a}\rfloor}_{n\in A}$ contains arbitrarily long arithmetic progressions for every $1\le a<2$ and every $A\subset\mathbb{N}$ with positive upper density. Using this corollary, we show that the set ${ \lfloor{\lfloor{p^{1/b}}\rfloor^a}\rfloor \mid \text{$p$ prime} }$ contains arbitrarily long arithmetic progressions for every $1\le a<2$ and $b>1$. We also prove that, for every $a\ge2$, the graph of ${\lfloor{n^a}\rfloor}_{n=1}^\infty$ does not contain any arithmetic progressions of length $3$.