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 A015921 #33 Sep 22 2016 20:15:27 %S A015921 1,2,4,6,10,12,14,22,26,30,34,38,46,58,62,74,82,86,94,106,118,122,132, %T A015921 134,142,146,158,166,170,178,182,194,202,206,214,218,226,254,262,274, %U A015921 278,298,302,314,326,334,346,358,362,372,382,386,394,398,422,446 %N A015921 Positive integers n such that 2^n == 4 (mod n). %C A015921 Odd terms are given by A173572. %C A015921 For all m, 2^A050259(m)-1 belongs to this sequence. %H A015921 T. D. Noe, <a href="/A015921/b015921.txt">Table of n, a(n) for n = 1..1000</a> %H A015921 OEIS Wiki, <a href="/wiki/2^n mod n">2^n mod n</a> %t A015921 Select[Range[500], PowerMod[2, #, #] == 4 &] (* _Alonso del Arte_, Jul 07 2011 *) %Y A015921 Contains A050990 as a subsequence. %Y A015921 Cf. A015910, A173572. %K A015921 nonn %O A015921 1,2 %A A015921 _Robert G. Wilson v_ %E A015921 Edited and terms 1,2,4 prepended by _Max Alekseyev_, Jul 29 2011