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 A291855 #24 Sep 13 2017 09:53:42 %S A291855 28,52,68,92,124,156,172,244,260,308,327,340,348,356,380,396,404,428, %T A291855 436,500,508,516,532,580,612,644,660,684,696,724,732,748,764,780,796, %U A291855 820,836,908,940,980,996,1056,1076,1124,1172,1180 %N A291855 Numbers k such that gpf(2^k - 1) - 1 is not divisible by k. %C A291855 For a(n) <= 10^3, all terms are divisible by 4 except 327 = 3*109. %C A291855 Primes p such that 4*p is a term are 7, 13, 17, 23, 31, 43, 61, ... %C A291855 From _Thomas Ordowski_, Sep 04 2017: (Start) %C A291855 If p == +-1 (mod 8) and 2^p - 1 is prime, then 4*p is a term. %C A291855 Conjecture: 4 * A001153(m) for m > 3 is a subsequence. %C A291855 Primes q such that q-1 is a term are 29, 53, 157, 173, 349, 397, ... (End) %e A291855 28 is a term because gpf(2^28 - 1) == 15 (mod 28). %Y A291855 Cf. A000225, A005420, A006530. %Y A291855 Subsequence of A292199. %K A291855 nonn %O A291855 1,1 %A A291855 _Thomas Ordowski_ and _Altug Alkan_, Sep 04 2017 %E A291855 a(10)-a(41) from _Giovanni Resta_, Sep 04 2017 %E A291855 a(42)-a(46) from _Max Alekseyev_, Sep 13 2017