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 A038844 #22 Jun 20 2025 18:48:10
%S A038844 5,21,37,72,73,76,100,121,153,221,233,237,245,276,288,292,296,300,305,
%T A038844 333,336,341,348,352,357,380,381,397,445,448,461,465,472,492,545,557,
%U A038844 565,576,577,601,605,637,648,657,676,688,692,696,737,752,753,761,776
%N A038844 Numbers k for which 6*k+1 divides 2^k-1.
%C A038844 Apart from 5, all terms are in A045762, numbers k such that 2^k-1 is not prime. - _Michel Marcus_, Nov 12 2014
%C A038844 6*k + 1 is not necessarily a prime for k being a term of this sequence. - _Jianing Song_, Jun 20 2025
%H A038844 Harvey P. Dale, <a href="/A038844/b038844.txt">Table of n, a(n) for n = 1..1000</a>
%e A038844 For n=5, 2^5-1=31 is divisible by 6*5+1=31.
%t A038844 Select[Range[800],PowerMod[2,#,6#+1]==1&] (* _Harvey P. Dale_, Oct 24 2017 *)
%o A038844 (PARI) select( {is_A038844(n)=Mod(2,n*6+1)^n==1}, [1..999]) \\ _M. F. Hasler_, Aug 17 2021
%Y A038844 Cf. A045762, A188130 (subsequence of primes).
%K A038844 nonn
%O A038844 1,1
%A A038844 _Bill Gosper_
%E A038844 More terms from _Michel Marcus_, Nov 12 2014