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 A126091 #2 Mar 31 2012 14:40:14
%S A126091 1,4,2,7,3,8,6,11,7,12,10,15,11,16,14,19,15,20,18,23,19,24,22,27,23,
%T A126091 28,26,31,27,32,30,35,31,36,34,39,35,40,38,43,39,44,42,47,43,48,46,51,
%U A126091 47,52,50,55,51,56,54,59,55,60,58,63,59,64,62,67,63,68,66,71,67,72,70,75
%N A126091 a(1)=1;a(2)=4; for n>2, a(n)=a(n-2)+3 if n is already in the sequence, a(n)=a(n-2)+1 otherwise.
%t A126091 a={1,4};Do[If[MemberQ[a,n],b=a[[n-2]]+3,b=a[[n-2]]+1];AppendTo[a,b],{n,3,70}];a
%Y A126091 Cf. A080578, A080455-A080458.
%K A126091 nonn
%O A126091 1,2
%A A126091 _Zak Seidov_, Mar 11 2007