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## Acta Arithmetica

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## Solution to a problem of Luca, Menares and Pizarro-Madariaga

### Volume 210 / 2023

Acta Arithmetica 210 (2023), 391-401 MSC: Primary 11N05; Secondary 11N36, 11N37. DOI: 10.4064/aa230604-12-7 Published online: 12 September 2023

#### Abstract

Let $k\ge 2$ be a positive integer and $P^+(n)$ the greatest prime factor of a positive integer $n$ with convention $P^+(1)=1$. For any $\theta \in \left [\frac 1{2k},\frac {17}{32k}\right )$, set $$T_{k,\theta }(x)=\sum _{\substack {p_1\cdot \cdot \cdot p_k\le x\\ P^+(\gcd (p_1-1,\ldots ,p_k-1))\ge (p_1\cdot \cdot \cdot p_k)^\theta }}1,$$ where the $p$’s are primes. It is proved that $$T_{k,\theta }(x)\ll _{k}\frac {x^{1-\theta (k-1)}}{(\log x)^2},$$ which, together with the lower bound $$T_{k,\theta }(x)\gg _{k}\frac {x^{1-\theta (k-1)}}{(\log x)^2}$$ obtained by Wu in 2019, answers a 2015 problem of Luca, Menares and Pizarro-Madariaga on the exact order of magnitude of $T_{k,\theta }(x)$.

A main novelty in the proof is that, instead of using the Brun–Titchmarsh theorem to estimate the $k$th moment of primes in arithmetic progressions, we give a transformation of this moment so that the task can be reduced to estimating certain clusters of primes.

#### Authors

• Yuchen DingSchool of Mathematical Sciences
Yangzhou University
Yangzhou 225002, P.R. China
e-mail
• Lilu ZhaoSchool of Mathematics
Shandong University
Jinan 250100, P.R. China
e-mail

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