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 A068628 #9 Jun 17 2017 03:45:52
%S A068628 0,60,216,468,816,1260,1800,2436,3168,3996,4920,5940,7056,8268,9576,
%T A068628 10980,12480,14076,15768,17556,19440,21420,23496,25668,27936,30300,
%U A068628 32760,35316,37968,40716,43560,46500,49536,52668,55896,59220,62640,66156,69768,73476
%N A068628 Numbers occurring twice in A068627.
%C A068628 Recall that A068627 is defined as follows: a(0) = 0, a(n) = a(n-1) + n if n does not divide a(n-1), a(n) = a(n-1) - n if n divides a(n-1), a(n) = n if a(n-1) = 0.
%C A068628 A068627 begins: 0,1,3,0,4,9,15,22,30,39,49,60,48,61,75,60,...
%C A068628 Numbers occurring twice in A068627, like 0, 60 etc. are called nodes.
%H A068628 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3,-3,1).
%o A068628 (PARI) already(val, v, nb)=for (i = 1, nb, if (val == v[i], return (1));); return (0);
%o A068628 lista(nn) = {preca = 0; vm = vector(nn); for (n=1, nn, vm[n] = preca; if (preca == 0, nexta = n, if (preca % n, nexta = preca + n, nexta = preca - n);); if (already(preca, vm, n-1), print1(preca, ", ")); preca = nexta;);} \\ _Michel Marcus_, Jan 23 2014
%K A068628 easy,nonn
%O A068628 1,2
%A A068628 _Amarnath Murthy_, Feb 26 2002
%E A068628 Name and comments edited by _Michel Marcus_, Jan 23 2014
%E A068628 Offset changed to 1 and more terms from _Michel Marcus_, Jan 23 2014