%I #13 Jul 30 2023 01:38:18

%S 2,31,103,617,10253,10691,35353,129587,556441

%N Numbers k such that (10^k - 7^k)/3 is prime.

%C All terms are prime.

%C The corresponding primes: 17, 3333280741539321718064461652419, ...

%H Jon Grantham and Andrew Granville, <a href="https://arxiv.org/abs/2307.07894">Fibonacci primes, primes of the form 2^n-k and beyond</a>, arXiv:2307.07894 [math.NT], 2023.

%t Select[Range[1, 10000], PrimeQ[(10^# - 7^#)/3] &]

%o (PARI) for(n=1, 10000, if(isprime((10^n - 7^n)/3), print1(n, ", ")))

%Y Cf. A004023, A128026, A062576.

%K nonn,more,hard

%O 1,1

%A _Tim Johannes Ohrtmann_, May 21 2016

%E a(7)-a(9) from _Jon Grantham_, Jul 29 2023