The Bose-Chowla argument for Sidon sets (2104.12711v3)

Abstract: Let $h \geq 2$ and let ${ \mathcal A} = (A_1,\ldots, A_h)$ be an $h$-tuple of sets of integers. For nonzero integers $c_1,\ldots, c_h$, consider the linear form $\varphi = c_1 x_1 + c_2x_2 + \cdots + c_h x_h$. The \emph{representation function} $R_{ \mathcal{A},\varphi}(n)$ counts the number of $h$-tuples $(a_1,\ldots, a_h) \in A_1 \times \cdots \times A_h$ such that $\varphi(a_1,\ldots, a_h) = n$. The $h$-tuple $\mathcal{A}$ is a \emph{$\varphi$-Sidon system of multiplicity $g$} if $R_{\mathcal A,\varphi}(n) \leq g$ for all $n \in \mathbf{Z}$. For every positive integer $g$, let $F_{\varphi,g}(n)$ denote the largest integer $q$ such that there exists a $\varphi$-Sidon system $\mathcal {A} = (A_1,\ldots, A_h)$ of multiplicity $g$ with [ A_i \subseteq [1,n] \qquad \text{and} \qquad |A_i| = q ] for all $i =1,\ldots, h$. It is proved that, for all linear forms $\varphi$, [ \limsup_{n\rightarrow \infty} \frac{F_{\varphi,g}(n)}{n^{1/h}} < \infty ] and, for linear forms $\varphi$ whose coefficients $c_i$ satisfy a certain divisibility condition, [ \liminf_{n\rightarrow\infty} \frac{F_{\varphi,h!}(n)}{n^{1/h}} \geq 1. ]