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 A011861 #22 Sep 08 2022 08:44:37
%S A011861 0,0,0,0,1,2,3,5,7,9,11,13,16,19,22,26,30,34,38,42,47,52,57,63,69,75,
%T A011861 81,87,94,101,108,116,124,132,140,148,157,166,175,185,195,205,215,225,
%U A011861 236,247,258,270,282,294,306,318,331,344,357,371,385,399,413,427,442,457,472
%N A011861 a(n) = floor(n(n-1)/8).
%H A011861 Matthew House, <a href="/A011861/b011861.txt">Table of n, a(n) for n = 0..10000</a>
%H A011861 <a href="/index/Rec#order_09">Index entries for linear recurrences with constant coefficients</a>, signature (3, -4, 4, -4, 4, -4, 4, -3, 1).
%F A011861 From _R. J. Mathar_, Apr 15 2010: (Start)
%F A011861 a(n) = +3*a(n-1) -4*a(n-2) +4*a(n-3) -4*a(n-4) +4*a(n-5) -4*a(n-6) +4*a(n-7) -3*a(n-8) +a(n-9).
%F A011861 G.f.: x^4*(x^2+1-x)/ ((1-x)^3 * (x^2+1) * (x^4+1)). (End)
%p A011861 seq(floor(binomial(n,2)/4), n=0..51); # _Zerinvary Lajos_, Jan 12 2009
%t A011861 LinearRecurrence[{3, -4, 4, -4, 4, -4, 4, -3, 1}, {0, 0, 0, 0, 1, 2, 3, 5, 7}, 70] (* _Vincenzo Librandi_, Aug 08 2015 *)
%o A011861 (Magma) [n*(n-1) div 8: n in [0..70]]; // _Vincenzo Librandi_, Aug 08 2015
%K A011861 nonn
%O A011861 0,6
%A A011861 _N. J. A. Sloane_