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 A309743 #19 Jan 07 2023 12:24:48 %S A309743 0,1,2,6,12,245,298,2967,4321,7225,11267 %N A309743 Numbers k such that 9*10^(2*k) + 9*10^k + 1 is prime. %C A309743 a(12) > 2*10^4. - _Tyler Busby_, Jan 07 2023 %e A309743 19 is prime. ==> a(1) = 0. %e A309743 991 is prime. ==> a(2) = 1. %e A309743 90901 is prime. ==> a(3) = 2. %e A309743 9009001 = 131 * 68771. %e A309743 900090001 = 421 * 2137981. %e A309743 90000900001 = 131 * 701 * 980071. %e A309743 9000009000001 is prime. ==> a(4) = 6. %o A309743 (PARI) for(k=0, 1e3, if(ispseudoprime(9*100^k+9*10^k+1), print1(k", "))) %Y A309743 Cf. A309739. %K A309743 nonn,more %O A309743 1,3 %A A309743 _Seiichi Manyama_, Aug 15 2019 %E A309743 a(10)-a(11) from _Tyler Busby_, Jan 07 2023