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 A080901 #11 Mar 03 2016 15:30:07
%S A080901 2,0,5,10,8,13,18,16,21,19,24,29,27,32,37,35,40,38,36,41,39,44,49,47,
%T A080901 52,57,55,60,58,63,68,66,71,76,74,72,70,68,66,64,62,67,72,70,75,80,78,
%U A080901 83,81,86,91,89,94,99,97,102,100,98,103,101,106,104,102,100
%N A080901 a(1)=2; for n>1, a(n)=a(n-1)-2 if n is already in the sequence, a(n)=a(n-1)+5 otherwise.
%H A080901 Ivan Neretin, <a href="/A080901/b080901.txt">Table of n, a(n) for n = 1..1008</a>
%H A080901 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 A080901 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.
%F A080901 a(n)-n (n >= 1) (A081745) is periodic with period 168.
%t A080901 Fold[Append[#1, #1[[-1]] + If[MemberQ[#1, #2], -2, 5]] &, {2}, Range[2, 64]] (* _Ivan Neretin_, Mar 03 2016 *)
%Y A080901 Cf. A080578, A080900, A081745.
%K A080901 nonn,easy
%O A080901 1,1
%A A080901 _N. J. A. Sloane_ and _Benoit Cloitre_, Apr 01 2003