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 A011865 #21 Sep 02 2024 08:37:34
%S A011865 0,0,0,0,1,1,2,3,4,6,7,9,11,13,15,17,20,22,25,28,31,35,38,42,46,50,54,
%T A011865 58,63,67,72,77,82,88,93,99,105,111,117,123,130,136,143,150,157,165,
%U A011865 172,180,188,196,204,212,221,229,238,247,256,266,275,285,295,305,315,325,336
%N A011865 a(n) = floor( n*(n - 1)/12 ).
%H A011865 Paolo Xausa, <a href="/A011865/b011865.txt">Table of n, a(n) for n = 0..10000</a>
%H A011865 <a href="/index/Rec#order_11">Index entries for linear recurrences with constant coefficients</a>, signature (2,-1,1,-2,1,-1,2,-1,1,-2,1).
%F A011865 From _R. J. Mathar_, Apr 15 2010: (Start)
%F A011865 a(n) = +2*a(n-1) -a(n-2) +a(n-3) -2*a(n-4) +a(n-5) -a(n-6) +2*a(n-7) -a(n-8) +a(n-9) -2*a(n-10) +a(n-11).
%F A011865 G.f.: x^4*(x^4+1-x^3-x+x^2) / ((1-x)^3*(1+x+x^2)*(x^2+1)*(x^4-x^2+1)). (End)
%p A011865 seq(floor(binomial(n-1,2)/6), n=1..53); # _Zerinvary Lajos_, Jan 12 2009
%t A011865 Table[Floor[n*(n - 1)/12], {n, 0, 100}] (* _Paolo Xausa_, Sep 02 2024 *)
%K A011865 nonn,easy
%O A011865 0,7
%A A011865 _N. J. A. Sloane_