This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A309719 #31 Sep 04 2024 15:22:39 %S A309719 0,1,3,6,12,2555,3281,5292,11209 %N A309719 Numbers k such that 64*10^(2*k) + 8*10^k + 1 is prime. %e A309719 73 is prime ==> a(1) = 0. %e A309719 6481 is prime ==> a(2) = 1. %e A309719 640801 = 7 * 31 * 2953. %e A309719 64008001 is prime ==> a(3) = 3. %e A309719 6400080001 = 7 * 13441 * 68023. %e A309719 640000800001 = 619 * 1033926979. %e A309719 64000008000001 is prime ==> a(4) = 6. %o A309719 (PARI) for(k=0, 1e3, if(ispseudoprime(64*100^k+8*10^k+1), print1(k", "))) %Y A309719 Cf. A309738, A309742. %K A309719 nonn,more,base %O A309719 1,3 %A A309719 _Seiichi Manyama_, Aug 15 2019 %E A309719 a(9) from _Michael S. Branicky_, Sep 04 2024