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 A171590 #15 Apr 04 2024 09:49:48
%S A171590 1,25,49,289,961,4225,16129,66049,261121,1050625,4190209,16785409,
%T A171590 67092481,268468225,1073676289,4295098369,17179607041,68720001025,
%U A171590 274876858369,1099513724929,4398042316801,17592194433025,70368727400449
%N A171590 a(n) = 1+4^(n+1)-4*(-2)^n.
%H A171590 Vincenzo Librandi, <a href="/A171590/b171590.txt">Table of n, a(n) for n = 0..1000</a>
%H A171590 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3,6,-8).
%F A171590 a(n) = A105951(2n+1) = (A014551(n+1))^2.
%F A171590 G.f.: (1+22*x-32*x^2)/((x-1)*(2*x+1)*(4*x-1)).
%F A171590 a(n) = 3*a(n-1)+6*a(n-2)-8*a(n-3).
%t A171590 CoefficientList[Series[(1 + 22*x - 32*x^2)/((x - 1)*(2*x + 1)*(4*x-1)), {x, 0, 40}], x] (* _Vincenzo Librandi_, Dec 19 2012 *)
%o A171590 (Magma) I:=[1, 25, 49]; [n le 3 select I[n] else 3*Self(n-1) + 6*Self(n-2) - 8*Self(n-3): n in [1..30]]; // _Vincenzo Librandi_, Dec 19 2012
%K A171590 easy,nonn
%O A171590 0,2
%A A171590 _R. J. Mathar_, Dec 12 2009
%E A171590 G.f. Adapted by _Vincenzo Librandi_, Dec 19 2012