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 A104686 #14 Aug 21 2022 17:49:34
%S A104686 0,1,3,0,4,3,3,4,0,3,1,0,0,1,3,0,4,3,3,4,0,3,1,0,0,1,3,0,4,3,3,4,0,3,
%T A104686 1,0,0,1,3,0,4,3,3,4,0,3,1,0,0,1,3,0,4,3,3,4,0,3,1,0,0,1,3,0,4,3,3,4,
%U A104686 0,3,1,0,0,1,3,0,4,3,3,4,0,3,1,0,0,1,3
%N A104686 a(n) = n*(n+1)/2 (mod 6).
%H A104686 <a href="/index/Rec#order_09">Index entries for linear recurrences with constant coefficients</a>, signature (0,0,1,0,0,-1,0,0,1)
%F A104686 a(n) = A010875(A000217(n)). - _Arkadiusz Wesolowski_, Nov 28 2011
%F A104686 G.f.: -x*(1+3*x+3*x^3+3*x^5+x^6) / ( (x-1)*(1+x+x^2)*(x^2+1)*(x^4-x^2+1) ). - _R. J. Mathar_, Oct 16 2015
%t A104686 Table[Mod[(n^2 + n)/2, 6], {n, 0, 100}] (* _Arkadiusz Wesolowski_, Nov 28 2011 *)
%t A104686 Mod[Accumulate[Range[0,90]],6] (* or *) LinearRecurrence[{0,0,1,0,0,-1,0,0,1},{0,1,3,0,4,3,3,4,0},90] (* _Harvey P. Dale_, Aug 21 2022 *)
%K A104686 easy,nonn
%O A104686 0,3
%A A104686 Stuart M. Ellerstein (ellerstein(AT)aol.com), Apr 22 2005