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 A216263 #22 Aug 06 2021 05:10:05
%S A216263 1,6,27,110,429,1638,6187,23238,87021,325358,1215435,4538430,16942381,
%T A216263 63239286,236031147,880918070,3287706669,12270039678,45792714187,
%U A216263 170901341358,637813699821,2380355555078,8883612714795,33154103692710,123732818833261,461777205194766,1723376069054667
%N A216263 Expansion of 1 / ((1-2*x)*(1-4*x+x^2)).
%H A216263 Colin Barker, <a href="/A216263/b216263.txt">Table of n, a(n) for n = 0..1000</a>
%H A216263 László Németh and László Szalay, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL24/Nemeth/nemeth8.html">Sequences Involving Square Zig-Zag Shapes</a>, J. Int. Seq., Vol. 24 (2021), Article 21.5.2.
%H A216263 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (6,-9,2).
%F A216263 G.f.: 1/((1-2*x)*(1-4*x+x^2)).
%F A216263 a(n) = 6*a(n-1) - 9*a(n-2) + 2*a(n-3), a(0) = 1, a(1) = 6, a(2) = 27.
%F A216263 3*a(n) = -2^(n+2) + A001075(n+2). - _R. J. Mathar_, Mar 29 2013
%F A216263 a(n) = (-2^(3+n) + (7-4*sqrt(3))*(2-sqrt(3))^n + (2+sqrt(3))^n*(7+4*sqrt(3))) / 6. - _Colin Barker_, Feb 05 2017
%t A216263 CoefficientList[Series[1/((1 - 2 x)*(1 - 4 x + x^2)), {x, 0, 26}], x] (* _Michael De Vlieger_, Aug 05 2021 *)
%o A216263 (PARI) Vec(1/((1-2*x)*(1-4*x+x^2)) + O(x^30)) \\ _Colin Barker_, Feb 05 2017
%Y A216263 A diagonal of A214846.
%Y A216263 Cf. A001075.
%K A216263 nonn,easy
%O A216263 0,2
%A A216263 _Philippe Deléham_, Mar 15 2013