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 A108320 #17 Oct 29 2024 14:17:20 %S A108320 1,2,3,4,5,6,12,17,43,46,50,88,110,187,472,832,834,1034,1311,1700, %T A108320 2756,5470,6124,7404,7891,12010,16411,24576,25461,27982,38762 %N A108320 Numbers k such that 609 * 10^k - 1 is prime. %e A108320 a(4) = 4 because 609*10^4-1 = 6089999, a prime. %o A108320 (PARI) is(n)=ispseudoprime(609*10^n-1) \\ _Charles R Greathouse IV_, Jun 13 2017 %Y A108320 Cf. A053580. %K A108320 more,nonn %O A108320 1,2 %A A108320 _Jason Earls_, Jun 30 2005 %E A108320 a(27) from _Michael S. Branicky_, May 19 2023 %E A108320 a(28)-a(31) from _Michael S. Branicky_, Oct 29 2024