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 A231372 #16 Feb 16 2025 08:33:20 %S A231372 119,133,161,203,323,329,391,413,427,437,493,551,667,679,763,791,799, %T A231372 893,917,1003,1037,1043,1081,1121,1159,1169,1253,1267,1351,1357,1363, %U A231372 1403,1561,1603,1631,1649,1711,1769,1799,1841,1843,1853,1883,1921,2071,2147,2191 %N A231372 Squarefree composite numbers k such that 10 is a primitive root for all prime factors of k. %C A231372 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 A231372 Amiram Eldar, <a href="/A231372/b231372.txt">Table of n, a(n) for n = 1..10000</a> %H A231372 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PrimitiveRoot.html">Primitive Root</a>. %H A231372 Wikipedia, <a href="http://en.wikipedia.org/wiki/Decimal">Decimal</a>. %t A231372 q[n_] := CompositeQ[n] && SquareFreeQ[n] && AllTrue[FactorInteger[n][[;;,1]], MultiplicativeOrder[10, #] == # - 1 &]; Select[Range[2000], q] (* _Amiram Eldar_, Oct 03 2021 *) %Y A231372 Subsequence of A024556. %Y A231372 Cf. A001913, A231370, A231371. %K A231372 nonn %O A231372 1,1 %A A231372 _Arkadiusz Wesolowski_, Nov 08 2013