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 A011920 #19 Feb 07 2024 11:55:29
%S A011920 12,1980,378840,73419192,14241916260,2762844014580,535977297450672,
%T A011920 103976830083273840,20170969020163148220,3913064012542622257452,
%U A011920 759114247456742016195720,147264250942490855924510760
%N A011920 a(n) = b(n)*(b(n)+1) = b(n) + ... + c(n), where b(n) = A011916(n), c(n) = A011918(n).
%D A011920 Mario Velucchi "Seeing couples" in Recreational and Educational Computing, to appear 1997.
%H A011920 Harvey P. Dale, <a href="/A011920/b011920.txt">Table of n, a(n) for n = 1..437</a>
%H A011920 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (209,-2926,2926,-209,1).
%F A011920 From _R. J. Mathar_, Apr 15 2010: (Start)
%F A011920 a(n) = +209*a(n-1) -2926*a(n-2) +2926*a(n-3) -209*a(n-4) +a(n-5).
%F A011920 G.f.: -12*x*(1-44*x+11*x^2)/ ((x-1) * (x^2-14*x+1) * (x^2-194*x+1)). (End)
%p A011920 A011922 := proc(n) (2+sqrt(1+((((2+sqrt(3))^(2*n)-(2-sqrt(3))^(2*n))^2)/4)))/3 ; expand(%) ; simplify(%) ; end proc:
%p A011920 A011916 := proc(n) ((A011922(n)-1)+sqrt(3*A011922(n)^2-4*A011922(n)+1))/2 ; end proc:
%p A011920 A011920 := proc(n) A011916(n)*(A011916(n)+1) ; end proc:
%p A011920 seq(A011920(n),n=1..20) ; # _R. J. Mathar_, Apr 15 2010
%t A011920 LinearRecurrence[{209,-2926,2926,-209,1},{12,1980,378840,73419192,14241916260},20] (* _Harvey P. Dale_, Jan 01 2021 *)
%K A011920 nonn,easy
%O A011920 1,1
%A A011920 Mario Velucchi (mathchess(AT)velucchi.it)
%E A011920 More terms from _R. J. Mathar_, Apr 15 2010