A108240 - OEIS

%I #14 Mar 29 2023 09:18:29

%S 0,1,2,7,45,54,60,99,195,271,476,1491,2713,8155

%N Numbers k such that (10^k)^2 + 5*(10^k) - 1 is prime.

%C All of the primes for the terms <= 2713 have been certified. Primality proof for the largest: PFGW Version 20041001.Win_Stable (v1.2 RC1b) [FFT v23.8] Primality testing (10^2713)^2+5*(10^2713)-1 [N+1, Brillhart-Lehmer-Selfridge] Running N+1 test using discriminant 7, base 1+sqrt(7) Calling Brillhart-Lehmer-Selfridge with factored part 34.96% (10^2713)^2+5*(10^2713)-1 is prime! (216.4510s+10.6592s)

%o (PARI) is(n)=ispseudoprime((10^n)^2+5*(10^n)-1) \\ _Charles R Greathouse IV_, Jun 13 2017

%Y Cf. A108195.

%K more,nonn

%O 1,3

%A _Jason Earls_, Jun 17 2005

%E a(14) from _Michael S. Branicky_, Mar 27 2023