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 A309741 #18 Sep 03 2024 15:02:48 %S A309741 0,1,2,4,22,133,567,14739,25390 %N A309741 Numbers k such that 6*10^(2*k) + 6*10^k + 1 is prime. %e A309741 13 is prime. ==> a(1) = 0. %e A309741 661 is prime. ==> a(2) = 1. %e A309741 60601 is prime. ==> a(3) = 2. %e A309741 6006001 = 2027 * 2963. %e A309741 600060001 is prime. ==> a(4) = 4. %o A309741 (PARI) for(k=0, 1e3, if(ispseudoprime(6*100^k+6*10^k+1), print1(k", "))) %Y A309741 Cf. A309739. %K A309741 nonn,more,base %O A309741 1,3 %A A309741 _Seiichi Manyama_, Aug 15 2019 %E A309741 a(8)-a(9) from _Michael S. Branicky_, Sep 03 2024