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

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{pp is prime and pa(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={nP(n)<(1ϵ)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)={nx<n<y, P(n)<(1ϵ)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 yy\to\infty,

Fϵ(x,y)xyenϵ/(log(n)loglog(n))dn,F_\epsilon(x,y)\ll\int_x^y e^{-n^\epsilon/(\log(n)\log\log(n))}\,dn,

and

lim infnP(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.

Sources & referencesView supporting material

Primary source

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

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