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 A108240 #14 Mar 29 2023 09:18:29 %S A108240 0,1,2,7,45,54,60,99,195,271,476,1491,2713,8155 %N A108240 Numbers k such that (10^k)^2 + 5*(10^k) - 1 is prime. %C A108240 All of the primes for the terms <= 2713 have been certified. Primality proof for the largest: PFGW Version 20041001.Win_Stable (v1.2 RC1b) [FFT v23.8] Primality testing (10^2713)^2+5*(10^2713)-1 [N+1, Brillhart-Lehmer-Selfridge] Running N+1 test using discriminant 7, base 1+sqrt(7) Calling Brillhart-Lehmer-Selfridge with factored part 34.96% (10^2713)^2+5*(10^2713)-1 is prime! (216.4510s+10.6592s) %o A108240 (PARI) is(n)=ispseudoprime((10^n)^2+5*(10^n)-1) \\ _Charles R Greathouse IV_, Jun 13 2017 %Y A108240 Cf. A108195. %K A108240 more,nonn %O A108240 1,3 %A A108240 _Jason Earls_, Jun 17 2005 %E A108240 a(14) from _Michael S. Branicky_, Mar 27 2023