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 A171557 #13 Jan 22 2017 21:57:37
%S A171557 1,3,-9,-81,-243,-1215,-5103,-15309,-59049,-216513,-649539,-2302911,
%T A171557 -7971615,-23914845,-81310473,-272629233,-817887699,-2711943423,
%U A171557 -8910671247,-26732013741,-87169610025,-282429536481,-847288609443
%N A171557 a(n) = 3^n*A168053(n).
%C A171557 Hankel transform of A171556.
%H A171557 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (3,0,27,-81).
%F A171557 G.f.: (1-18*x^2-81*x^3)/((1-3*x)^2*(1+3*x+9*x^2)).
%t A171557 CoefficientList[Series[(1 - 18 x^2 - 81 x^3)/((1 - 3 x)^2*(1 + 3 x + 9 x^2)), {x, 0, 30}], x] (* _Wesley Ivan Hurt_, Jan 21 2017 *)
%o A171557 (PARI) Vec((1-18*x^2-81*x^3)/((1-3*x)^2*(1+3*x+9*x^2)) + O(x^30)) \\ _Michel Marcus_, Feb 10 2015
%Y A171557 Cf. A168053, A171556.
%K A171557 easy,sign
%O A171557 0,2
%A A171557 _Paul Barry_, Dec 11 2009