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 A309742 #24 Apr 18 2023 10:20:47 %S A309742 0,1,6,11,23,297,474,1121,2531,3573,5437,5919 %N A309742 Numbers k such that 8*10^(2*k) + 8*10^k + 1 are prime. %e A309742 17 is prime. ==> a(1) = 0. %e A309742 881 is prime. ==> a(2) = 1. %e A309742 80801 = 7^2 * 17 * 97. %e A309742 8008001 = 47 * 170383. %e A309742 800080001 = 7 * 23 * 103 * 48247. %e A309742 80000800001 = 71 * 1126771831. %e A309742 8000008000001 is prime. ==> a(3) = 6. %o A309742 (PARI) for(k=0, 1e3, if(ispseudoprime(8*100^k+8*10^k+1), print1(k", "))) %o A309742 (Python) %o A309742 from sympy import isprime %o A309742 def afind(limit, startk=0): %o A309742 for k in range(startk, limit+1): %o A309742 if isprime(8*100**k + 8*10**k + 1): print(k, end=", ") %o A309742 afind(500) # _Michael S. Branicky_, Dec 12 2021 %Y A309742 Cf. A309739. %K A309742 nonn,more %O A309742 1,3 %A A309742 _Seiichi Manyama_, Aug 15 2019 %E A309742 a(11) from _Michael S. Branicky_, Dec 12 2021 %E A309742 a(12) from _Michael S. Branicky_, Apr 16 2023