Open-access mathematical research insights
About Contact
Home / Erdos Problems / Problem #783

Problem #783: Fix some constant $C>0$ and let $n$ be large

Fix some constant $C>0$ and let $n$ be large. Let $A\subseteq \{2,\ldots,n\}$ be such that $(a,b)=1$ for all $a\neq b\in A$ and $\sum_{n\in...

Problem Statement

Fix some constant $C>0$ and let $n$ be large. Let $A\subseteq \{2,\ldots,n\}$ be such that $(a,b)=1$ for all $a\neq b\in A$ and $\sum_{n\in A}\frac{1}{n}\leq C$.

What choice of such an $A$ minimises the number of integers $m\leq n$ not divisible by any $a\in A$? Is this minimised by letting $n\geq q_1>q_2>\cdots$ be the consecutive primes in decreasing order and choosing $A=\{q_1,\ldots,q_k\}$ where $k$ is maximal such that\[\sum_{i=1}^k\frac{1}{q_i}\leq C?\]
Categories: Number Theory

Progress

Source: erdosproblems.com/783 | Last verified: January 16, 2026

Stay Updated

Get weekly digests of new research insights delivered to your inbox.