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 A309740 #18 Sep 03 2024 16:33:41 %S A309740 0,3,5,301,13817,15259 %N A309740 Numbers k such that 5*10^(2*k) + 5*10^k + 1 is prime. %C A309740 11 | 5*10^(4*m) + 5*10^(2*m) + 1. So a(n) is odd for n > 1. %e A309740 11 is prime ==> a(1) = 0. %e A309740 551 = 19 * 29. %e A309740 50501 = 11 * 4591. %e A309740 5005001 is prime ==> a(2) = 3. %e A309740 500050001 = 11 * 61 * 745231. %e A309740 50000500001 is prime ==> a(3) = 5. %e A309740 5000005000001 = 11 * 31 * 1801 * 8141461. %o A309740 (PARI) for(k=0, 1e3, if(ispseudoprime(5*100^k+5*10^k+1), print1(k", "))) %Y A309740 Cf. A309739. %K A309740 nonn,base,more %O A309740 1,2 %A A309740 _Seiichi Manyama_, Aug 15 2019 %E A309740 a(5)-a(6) from _Michael S. Branicky_, Sep 03 2024