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 A104009 #28 Aug 12 2025 02:19:23
%S A104009 0,12,1848,351780,68149872,13219419708,2564481115560,497495864091732,
%T A104009 96511629630137568,18722758603319903340,3632118656731075949592,
%U A104009 704612296637707504759428,136691153428925957468727120,26517379152913151616036727452,5144234864511696770165553009288
%N A104009 Area of (a,a,a+1)-integer triangle. Corresponding a's are in A103974.
%H A104009 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (208,-2718,208,-1).
%F A104009 s(n) = ((a+1)/4)*sqrt((3*a+1)*(a-1)) where a=A103974(n).
%F A104009 a(n+1) = (((2 + sqrt(3))^(2*n) - (2 - sqrt(3))^(2*n))*(4 + (2 - sqrt(3))^(2*n) + (2 + sqrt(3))^(2*n)))/(12*sqrt(3)). - _Terentyev Oleg_, Nov 15 2009
%F A104009 G.f.: 12*x^2*(x^2-54*x+1) / ((x^2-194*x+1)*(x^2-14*x+1)). - _Colin Barker_, Apr 10 2013
%t A104009 Block[{a, nmax = 14}, a[n_] := ((-(2 - Sqrt[3])^(2 n) + (2 + Sqrt[3])^( 2 n)) (4 + (2 - Sqrt[3])^(2 n) + (2 + Sqrt[3])^(2 n)))/( 12 Sqrt[3]); Expand[a /@ Range[0, nmax]]] (* _Terentyev Oleg_, Nov 15 2009 *)
%t A104009 LinearRecurrence[{208,-2718,208,-1},{0,12,1848,351780},20] (* _Harvey P. Dale_, Mar 02 2018 *)
%Y A104009 Cf. A103974.
%K A104009 nonn,easy
%O A104009 1,2
%A A104009 _Zak Seidov_, Feb 24 2005
%E A104009 More terms from _Rick L. Shepherd_, Sep 05 2005
%E A104009 More terms from _Colin Barker_, Apr 10 2013