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 A308449 #31 Apr 17 2023 18:36:07 %S A308449 0,1,2,14,174,448,16562 %N A308449 Numbers k such that 25*10^(2*k) + 5*10^k + 1 are prime. %e A308449 31 is prime ==> a(1) = 0. %e A308449 2551 is prime ==> a(2) = 1. %e A308449 250501 is prime ==> a(3) = 2. %e A308449 25005001 = 7 * 79 * 103 * 439. %e A308449 2500050001 = 19 * 2269 * 57991. %e A308449 250000500001 = 7 * 103561 * 344863. %o A308449 (PARI) for(k=0, 1e3, if(ispseudoprime(25*100^k+5*10^k+1), print1(k", "))) %Y A308449 Cf. A309738. %K A308449 nonn,more,base %O A308449 1,3 %A A308449 _Seiichi Manyama_, Aug 15 2019 %E A308449 a(7) from _Michael S. Branicky_, Apr 17 2023