📝 SummarXiv

Abstract

For any $0<c<1$ let $$ T_c(x)=|\big\{p\le x: p\in \mathbb{P}, P^+(p-1)\ge p^c\big\}|, $$ where $\mathbb{P}$ is the set of primes and $P^+(n)$ denotes the largest prime factor of $n$. Erd\H os proved in 1935 that $$ \limsup_{x\rightarrow \infty}T_c(x)/\pi(x)\rightarrow 0, \quad \text{as~}c\rightarrow 1, $$ where $\pi(x)$ denotes the number of primes not exceeding $x$. Recently, Ding gave a quantitative form of Erd\H os' result and showed that for $8/9< c<1$ we have $$ \limsup_{x\rightarrow \infty}T_c(x)/\pi(x)\le 8\big(c^{-1}-1\big). $$ In this article, Ding's bound is improved to $$ \limsup_{x\rightarrow \infty}T_c(x)/\pi(x)\le -\frac{7}{2}\log c $$ for $e^{-\frac{2}{7}}< c<1$.