Multiple representations in difference sets under polynomial density constraints

Abstract

Motivated by the classical study of additive representation functions and the combinatorial phenomenon of sum-difference asymmetry, we investigate the subtractive representation function $d_{\mathcal{B},\mathcal{A}}(n)$, which counts the number of solutions of the equation $n = b - a$ with $a\in \mathcal{A}$ and $b\in \mathcal{B}$. Specifically, we study lower bounds and the asymptotic behavior of $d_{\mathcal{B},\mathcal{A}}(n)$ under polynomial density constraints of orders $x^α$ and $x^{1-α}$, where $0 < α< 1$. We first establish a general lower bound for the first moment of $d_{\mathcal{B},\mathcal{A}}(n)$. Furthermore, by employing a recursive construction, we prove that this lower bound is essentially optimal under certain regularity assumptions on the counting functions. Finally, utilizing probabilistic methods, we show the existence of sets for which $d_{\mathcal{B},\mathcal{A}}(n)$ has a logarithmic point-wise upper bound.