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 A127869 #24 Dec 18 2022 12:05:52
%S A127869 12,60,432,2348,13144,69280,361012,1841736,9286900,46303316,228903592,
%T A127869 1123242916,5477879120,26572232312,128302070508,616985221280,
%U A127869 2956362520140,14120605179500,67252176519008,319477138444252,1514116534887688,7160712605686480,33799490762646948
%N A127869 Number of L-shaped tiles in all tilings of a 3 X n board with 1 X 1 and L-shaped tiles (where the L-shaped tiles cover 3 squares).
%H A127869 P. Z. Chinn, R. Grimaldi and S. Heubach, <a href="http://www.cs.uwaterloo.ca/journals/JIS/VOL10/Heubach/heubach40.html">Tiling with Ls and Squares</a>, J. Int. Sequences 10 (2007) #07.2.8.
%H A127869 S. Heubach, <a href="https://www.calstatela.edu/sites/default/files/users/u1231/Presentations/talklv.pdf">Tiling with Ls and Squares</a>, 2005.
%H A127869 <a href="/index/Rec#order_08">Index entries for linear recurrences with constant coefficients</a>, signature (6,5,-44,-39,2,-29,4,-4).
%F A127869 G.f.: 4*x^2*(x-1)*(x^3-3*x^2-3)/(1-3*x-7*x^2+x^3-2*x^4)^2.
%e A127869 a(2) = 12 because the 3 X 2 board can be tiled in one way with only square tiles, in 8 ways using one L-tile and 3 square tiles and in 2 ways with 2 L-tiles, so there are altogether 8 + 2*2 = 12 L-tiles in all of the 3 X 2 tilings.
%t A127869 Table[Coefficient[Normal[Series[4x^2(3-3x+3x^2-4x^3+x^4)/(1-3x-7x^2+x^3-2x^4)^2, {x, 0, 30}]], x, n], {n, 0, 30}]
%Y A127869 Cf. A127864, A127865, A127866, A127867, A127868, A127870.
%K A127869 nonn
%O A127869 2,1
%A A127869 Silvia Heubach (sheubac(AT)calstatela.edu), Feb 03 2007