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Numbers k such that 64*10^(2*k) + 8*10^k + 1 is prime.
1

%I #31 Sep 04 2024 15:22:39

%S 0,1,3,6,12,2555,3281,5292,11209

%N Numbers k such that 64*10^(2*k) + 8*10^k + 1 is prime.

%e 73 is prime ==> a(1) = 0.

%e 6481 is prime ==> a(2) = 1.

%e 640801 = 7 * 31 * 2953.

%e 64008001 is prime ==> a(3) = 3.

%e 6400080001 = 7 * 13441 * 68023.

%e 640000800001 = 619 * 1033926979.

%e 64000008000001 is prime ==> a(4) = 6.

%o (PARI) for(k=0, 1e3, if(ispseudoprime(64*100^k+8*10^k+1), print1(k", ")))

%Y Cf. A309738, A309742.

%K nonn,more,base

%O 1,3

%A _Seiichi Manyama_, Aug 15 2019

%E a(9) from _Michael S. Branicky_, Sep 04 2024