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 A309744 #24 Sep 05 2024 10:16:16 %S A309744 1,3,17,31,7101,14183 %N A309744 Numbers k such that 81*10^(2*k) + 9*10^k + 1 is prime. %e A309744 91 = 7 * 13. %e A309744 8191 is prime ==> a(1) = 0. %e A309744 810901 = 7^2 * 13 * 19 * 67. %e A309744 81009001 is prime ==> a(2) = 3. %e A309744 8100090001 = 73 * 271 * 331 * 1237. %e A309744 810000900001 = 1471 * 550646431. %e A309744 81000009000001 = 7 * 13 * 613 * 757 * 1129 * 1699. %e A309744 8100000090000001 = 31 * 439 * 595194363289. %e A309744 810000000900000001 = 7 * 13 * 157 * 193 * 4243 * 4969 * 13933. %e A309744 81000000009000000001 = 271 * 298892988963099631. %o A309744 (PARI) for(k=0, 1e3, if(ispseudoprime(81*100^k+9*10^k+1), print1(k", "))) %Y A309744 Cf. A309738, A309743. %K A309744 nonn,more,base %O A309744 1,2 %A A309744 _Seiichi Manyama_, Aug 15 2019 %E A309744 a(6) from _Michael S. Branicky_, Sep 05 2024