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 A137990 #8 May 15 2023 12:15:00 %S A137990 2,7,19,109,163,487,1459,17497,52489,39367,472393,4960117,5314411, %T A137990 102036673,19131877,57395629,86093443,258280327,3874204891, %U A137990 23245229341,90656394427,585779779369,251048476873,9790890598009,4518872583697 %N A137990 Least prime p of the form c*3^n+1 with c not divisible by 3. %C A137990 a(n) is also the least prime such that 3^(n+1), but not 3^(n+2), divides 2^(a(n)-1)-1. %H A137990 Carlos Rivera, <a href="https://www.primepuzzles.net/puzzles/puzz_1129.htm">Puzzle 1129 OEIS A137990</a>, The Prime Puzzles & Problems Connection. %e A137990 a(8)=52489 because 52489=8*3^8+1 is prime and no smaller prime p has p-1 divisible by 3^8 but not 3^9. %Y A137990 Cf. A005109, A057775. %K A137990 easy,nonn %O A137990 0,1 %A A137990 _Andrew V. Sutherland_, May 01 2008