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 A231371 #18 Feb 16 2025 08:33:20 %S A231371 15,33,55,87,145,159,165,177,249,265,295,303,319,321,393,415,435,447, %T A231371 505,519,535,537,583,591,649,655,681,745,795,807,865,879,885,895,913, %U A231371 951,957,985,1041,1111,1135,1167,1177,1245,1257,1329,1345,1383,1401,1441 %N A231371 Squarefree composite numbers k such that 8 is a primitive root for all prime factors of k. %C A231371 If k is the smallest integer satisfying 10^k == 1 (mod p), we say that 10 has order k (mod p). If n is the product of distinct primes p_i, the period of 1/n in base b is the least common multiple of the orders of b (mod p_i), provided b and n are relatively prime. %H A231371 Amiram Eldar, <a href="/A231371/b231371.txt">Table of n, a(n) for n = 1..10000</a> %H A231371 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PrimitiveRoot.html">Primitive Root</a>. %H A231371 Wikipedia, <a href="http://en.wikipedia.org/wiki/Octal">Octal</a>. %t A231371 q[n_] := CompositeQ[n] && SquareFreeQ[n] && AllTrue[FactorInteger[n][[;;,1]], MultiplicativeOrder[8, #] == # - 1 &]; Select[Range[1441], q] (* _Amiram Eldar_, Oct 03 2021 *) %Y A231371 Subsequence of A024556. %Y A231371 Cf. A019338, A231370, A231372. %K A231371 nonn %O A231371 1,1 %A A231371 _Arkadiusz Wesolowski_, Nov 08 2013