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 A137480 #10 Feb 23 2017 22:15:05
%S A137480 12,21,48,84,192,336,768,1344,3072,5376,12288,21504,49152,86016,
%T A137480 196608,344064,786432,1376256,3145728,5505024,12582912,22020096,
%U A137480 50331648,88080384,201326592,352321536,805306368,1409286144,3221225472,5637144576,12884901888
%N A137480 a(n)=4a(n-2).
%H A137480 G. C. Greubel, <a href="/A137480/b137480.txt">Table of n, a(n) for n = 0..1000</a>
%H A137480 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (0,4).
%F A137480 G.f.: (21*x + 12)/(-4*x^2 + 1).
%F A137480 a(n) = -(9/4)*sqrt(2)*(1/2)^(-1/2*(-1)^n)*(-2)^n + (33/4)*2^n*sqrt(2)*(1/2)^(-(1/2)*(-1)^n) - _Alexander R. Povolotsky_, Apr 25 2008
%F A137480 a(n) = 3*A133600(n+2) - _R. J. Mathar_, Jun 08 2016
%t A137480 LinearRecurrence[{0, 4}, {12, 21}, 50] (* _G. C. Greubel_, Feb 23 2017 *)
%o A137480 (PARI) x='x + O('x^50); Vec((21*x + 12)/(-4*x^2 + 1)) \\ _G. C. Greubel_, Feb 23 2017
%K A137480 nonn,easy
%O A137480 0,1
%A A137480 _Paul Curtz_, Apr 22 2008