Fiori's lower-bound conjecture for the least prime in an arithmetic progression

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Let nn be a positive integer, let a<na<n satisfy gcd⁡(a,n)=1\operatorname{gcd}(a,n)=1, and define

P(n,a)=min⁡{p∣p is prime and p≡a(modn)},P(n,a)=\min\{p\mid p\text{ is prime and }p\equiv a\pmod n\}, P(n)=max⁡(a,n)=1P(n,a),P(n)=\max_{(a,n)=1}P(n,a),

where ϕ\phi denotes Euler's totient function. For 0<ϵ<10<\epsilon<1, define

Fϵ={n∣P(n)<(1−ϵ)ϕ(n)log⁡(n)log⁡(ϕ(n))},F_\epsilon=\{n\mid P(n)<(1-\epsilon)\phi(n)\log(n)\log(\phi(n))\}, Fϵ(x,y)={n∣x<n<y, P(n)<(1−ϵ)ϕ(n)log⁡(n)log⁡(ϕ(n))}.F_\epsilon(x,y)=\{n\mid x<n<y,\ P(n)<(1-\epsilon)\phi(n)\log(n)\log(\phi(n))\}.

Fiori's lower-bound conjecture. For 0<ϵ<10<\epsilon<1, as y→∞y\to\infty,

Fϵ(x,y)≪∫xye−nϵ/(log⁡(n)log⁡log⁡(n)) dn,F_\epsilon(x,y)\ll\int_x^y e^{-n^\epsilon/(\log(n)\log\log(n))}\,dn,

and

lim inf⁡n→∞P(n)ϕ(n)log⁡(n)log⁡(ϕ(n))=1.\liminf_{n\to\infty}\frac{P(n)}{\phi(n)\log(n)\log(\phi(n))}=1.

In particular, FϵF_\epsilon is finite for every 0<ϵ<10<\epsilon<1. This complements the predicted upper behavior and is supported by the paper's numerical tables, but remains unproved in the supplied text.

References

Primary source

Andrew Fiori, “The Least Prime in Arithmetic an Progression”, arXiv:2404.02329 (2024).

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