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 A011862 #13 Jul 08 2025 02:40:59
%S A011862 0,0,0,0,1,2,3,4,6,8,10,12,14,17,20,23,26,30,34,38,42,46,51,56,61,66,
%T A011862 72,78,84,90,96,103,110,117,124,132,140,148,156,164,173,182,191,200,
%U A011862 210,220,230,240,250,261,272,283,294,306,318,330,342,354,367,380,393,406,420
%N A011862 a(n) = floor(n*(n-1)/9).
%H A011862 <a href="/index/Rec#order_11">Index entries for linear recurrences with constant coefficients</a>, signature (2,-1,0,0,0,0,0,0,1,-2,1).
%F A011862 From _R. J. Mathar_, Apr 15 2010: (Start)
%F A011862 G.f.: x^4*(1 + x^4)/ ((1 - x)^3*(1 + x + x^2)*(1 + x^3 + x^6)).
%F A011862 a(n) = 2*a(n-1) - a(n-2) + a(n-9) - 2*a(n-10) + a(n-11). (End)
%t A011862 Table[Floor[(n(n-1))/9],{n,0,70}] (* or *) LinearRecurrence[{2,-1,0,0,0,0,0,0,1,-2,1},{0,0,0,0,1,2,3,4,6,8,10},70] (* _Harvey P. Dale_, Oct 01 2017 *)
%K A011862 nonn,easy
%O A011862 0,6
%A A011862 _N. J. A. Sloane_