%I #18 Sep 03 2024 15:02:48
%S 0,1,2,4,22,133,567,14739,25390
%N Numbers k such that 6*10^(2*k) + 6*10^k + 1 is prime.
%e 13 is prime. ==> a(1) = 0.
%e 661 is prime. ==> a(2) = 1.
%e 60601 is prime. ==> a(3) = 2.
%e 6006001 = 2027 * 2963.
%e 600060001 is prime. ==> a(4) = 4.
%o (PARI) for(k=0, 1e3, if(ispseudoprime(6*100^k+6*10^k+1), print1(k", ")))
%Y Cf. A309739.
%K nonn,more,base
%O 1,3
%A _Seiichi Manyama_, Aug 15 2019
%E a(8)-a(9) from _Michael S. Branicky_, Sep 03 2024