A308449 - OEIS

%I #31 Apr 17 2023 18:36:07

%S 0,1,2,14,174,448,16562

%N Numbers k such that 25*10^(2*k) + 5*10^k + 1 are prime.

%e           31 is prime ==> a(1) = 0.

%e         2551 is prime ==> a(2) = 1.

%e       250501 is prime ==> a(3) = 2.

%e     25005001 = 7 * 79 * 103 * 439.

%e   2500050001 = 19 * 2269 * 57991.

%e 250000500001 = 7 * 103561 * 344863.

%o (PARI) for(k=0, 1e3, if(ispseudoprime(25*100^k+5*10^k+1), print1(k", ")))

%Y Cf. A309738.

%K nonn,more,base

%O 1,3

%A _Seiichi Manyama_, Aug 15 2019

%E a(7) from _Michael S. Branicky_, Apr 17 2023