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 A054147 #16 Sep 08 2022 08:45:00
%S A054147 0,3,21,108,492,2100,8604,34272,133728,513648,1948560,7318080,
%T A054147 27256896,100815936,370684608,1355996160,4938304512,17914202880,
%U A054147 64760732928,233390693376,838784916480,3006980379648,10755352869888
%N A054147 a(n) = T(2n,n), array T as in A054144.
%H A054147 G. C. Greubel, <a href="/A054147/b054147.txt">Table of n, a(n) for n = 0..1000</a>
%H A054147 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (8,-20,16,-4).
%F A054147 G.f.: 3*x*(1-x)/(1-4*x+2*x^2)^2.
%F A054147 From _Colin Barker_, Aug 01 2019: (Start)
%F A054147 a(n) = 8*a(n-1) - 20*a(n-2) + 16*a(n-3) - 4*a(n-4) for n>3.
%F A054147 a(n) = 3*((-(2-sqrt(2))^n*(-1+sqrt(2)) + (1+sqrt(2))*(2+sqrt(2))^n)*n) / 8.
%F A054147 (End)
%t A054147 LinearRecurrence[{8,-20,16,-4}, {0,3,21,108}, 30] (* _G. C. Greubel_, Jul 31 2019 *)
%o A054147 (PARI) my(x='x+O('x^30)); concat([0], Vec(3*x*(1-x)/(1-4*x+2*x^2)^2)) \\ _G. C. Greubel_, Jul 31 2019
%o A054147 (Magma) R<x>:=PowerSeriesRing(Integers(), 30); [0] cat Coefficients(R!( 3*x*(1-x)/(1-4*x+2*x^2)^2 )); // _G. C. Greubel_, Jul 31 2019
%o A054147 (Sage) (3*x*(1-x)/(1-4*x+2*x^2)^2).series(x, 30).coefficients(x, sparse=False) # _G. C. Greubel_, Jul 31 2019
%o A054147 (GAP) a:=[0,3,21,108];; for n in [5..30] do a[n]:=8*a[n-1]-20*a[n-2] +16*a[n-3]-4*a[n-4]; od; a; # _G. C. Greubel_, Jul 31 2019
%Y A054147 Cf. A054144.
%K A054147 nonn,easy
%O A054147 0,2
%A A054147 _Clark Kimberling_, Mar 18 2000