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 A066507 #18 Feb 10 2021 21:03:27
%S A066507 1,2,4,6,7,8,10,12,14,16,17,18,20,22,23,24,26,28,30,31,32,34,36,38,40,
%T A066507 41,42,44,46,47,48,49,50,52,54,56,58,60,62,64,66,68,70,71,72,73,74,76,
%U A066507 78,79,80,82,84,86,88,89,90,92,94,96,97,98,100,102,103,104,106,108,110
%N A066507 Numbers k such that there is a solution to x^2 == 2^k (mod k).
%C A066507 All even numbers are in this sequence.
%C A066507 Odd terms in the sequence are numbers whose prime factors are +-1 (mod 8) (A058529), i.e., odd k such that x^2 == 2 (mod k) has a solution. - _Jason Earls_, Jan 22 2002
%o A066507 (PARI) isok(n) = {for (x=0, n-1, if (Mod(x, n)^2 == Mod(2, n)^n, return (1));); return (0);} \\ _Michel Marcus_, Nov 20 2013
%Y A066507 Cf. A058529, A001132, A057126.
%K A066507 nonn
%O A066507 1,2
%A A066507 _Benoit Cloitre_, Jan 04 2002
%E A066507 Corrected by _Vladeta Jovovic_, Jan 22 2002
%E A066507 More terms from _Jason Earls_, Jan 22 2002