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Numbers k such that 6*10^(2*k) + 6*10^k + 1 is prime.
1

%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