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 A140507 #10 Oct 25 2018 11:09:14
%S A140507 5,10,11,15,16,17,18,19,20,22,23,25,27,28,29,30,31,32,33,34,35,36,37,
%T A140507 38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,
%U A140507 61,62,64,65,66,67,68,69,70,72,73,74,75,76,77,78,79,80,81,82,83,84,85
%N A140507 Numbers k such that 3^k-1 contains a divisor which is an overpseudoprime in base 3.
%C A140507 An odd prime p is in the sequence iff p is not in A028491.
%o A140507 (PARI) isokd(n) = (n!=1) && !isprime(n) && (gcd(n,3)==1) && (znorder(Mod(3,n)) * (sumdiv(n, d, eulerphi(d)/znorder(Mod(3, d))) - 1) + 1 == n); \\ A141350
%o A140507 isok(n) = {fordiv (3^n-1, d, if (isokd(d), return (1));); return (0);}
%Y A140507 Cf. A141350, A020229, A141232, A141390, A003462, A028491.
%K A140507 nonn
%O A140507 1,1
%A A140507 _Vladimir Shevelev_, Jun 30 2008
%E A140507 More terms from _Michel Marcus_, Oct 25 2018