On disjoint sets

TL;DR

This paper investigates properties of disjoint sets of nonnegative integers, extending prior work by Erd"H os and Freud, and establishes asymptotic behaviors of their counting functions under certain conditions.

Contribution

It proves new asymptotic results about the product of counting functions of disjoint sets, generalizing earlier constructions and answering open questions.

Findings

If A(x)B(x)/x approaches 2, then A(y)B(y)/y approaches 1 within certain ranges.

For y between multiples of x, A(y)B(y) approximates 2x with small error.

The results describe the distribution and density behavior of disjoint sets under specific growth conditions.

Abstract

Two sets of nonnegative integers $A=\{a_1<a_2<\cdots\}$ and $B=\{b_1<b_2<\cdots\}$ are defined as \emph{disjoint}, if $\{A-A\}\bigcap\{B-B\}=\{0\}$, namely, the equation $a_i+b_t=a_j+b_k$ has only trivial solution. In 1984, Erd\H os and Freud [J. Number Theory 18 (1984), 99-109.] constructed disjoint sets $A,B$ with $A(x)>\varepsilon\sqrt{x}$ and $B(x)>\varepsilon\sqrt{x}$ for some $\varepsilon>0$, which answered a problem posed by Erd\H os and Graham. In this paper, following Erd\H{o}s and Freud's work, we explore further properties for disjoint sets. As a main result, we prove that, for disjoint sets $A$ and $B$, assume that $\{x_1<x_2<\cdots\}$ is a set of positive integers such that $\frac{A(x_n)B(x_n)}{x_n}\rightarrow 2$ as $x_n\to \infty$, then, (i) for any $0<c_1<c_2<1,$ $c_1x_n\le y\le c_2x_n$, we have $\frac{A(y)B(y)}{y}\rightarrow1$ as $n\rightarrow \infty$; (ii) for any $1<c_3<c_4<2,$ $c_3x_n\le y\le c_4x_n$, we have $A(y)B(y)=(2+o(1))x_n$ as $n\rightarrow \infty$.