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 A080904 #9 Mar 03 2016 15:28:12
%S A080904 1,6,11,16,21,17,22,27,32,37,33,38,43,48,53,49,45,50,55,60,56,52,57,
%T A080904 62,67,72,68,73,78,83,88,84,80,85,90,95,91,87,92,97,102,107,103,108,
%U A080904 104,109,114,110,106,102,107,103,99,104,100,96,92,97,102,98,103,99
%N A080904 a(1)=1; for n>1, a(n)=a(n-1)-4 if n is already in the sequence, a(n)=a(n-1)+5 otherwise.
%C A080904 a(n)-n is eventually periodic with period 18. - _Ivan Neretin_, Mar 03 2016
%H A080904 Ivan Neretin, <a href="/A080904/b080904.txt">Table of n, a(n) for n = 1..1000</a>
%H A080904 B. Cloitre, N. J. A. Sloane and M. J. Vandermast, <a href="http://www.cs.uwaterloo.ca/journals/JIS/VOL6/Cloitre/cloitre2.html">Numerical analogues of Aronson's sequence</a>, J. Integer Seqs., Vol. 6 (2003), #03.2.2.
%H A080904 B. Cloitre, N. J. A. Sloane and M. J. Vandermast, <a href="http://arXiv.org/abs/math.NT/0305308">Numerical analogues of Aronson's sequence</a>, arXiv:math/0305308 [math.NT], 2003.
%t A080904 Fold[Append[#1, #1[[-1]] + If[MemberQ[#1, #2], -4, 5]] &, {1}, Range[2, 62]] (* _Ivan Neretin_, Mar 03 2016 *)
%Y A080904 Cf. A080578, A080900.
%K A080904 nonn
%O A080904 1,2
%A A080904 _Benoit Cloitre_, Apr 01 2003