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a(1) = 4, a(2) = 12, for n>1: a(n) = 3*4^(n-1).
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%I #22 Jun 29 2023 11:32:12

%S 4,12,48,192,768,3072,12288,49152,196608,786432,3145728,12582912,

%T 50331648,201326592,805306368,3221225472,12884901888,51539607552,

%U 206158430208,824633720832,3298534883328,13194139533312,52776558133248

%N a(1) = 4, a(2) = 12, for n>1: a(n) = 3*4^(n-1).

%H Vincenzo Librandi, <a href="/A110594/b110594.txt">Table of n, a(n) for n = 1..1000</a>

%H <a href="/index/Rec#order_01">Index entries for linear recurrences with constant coefficients</a>, signature (4).

%F a(n) = A002001(n), n>1. - _R. J. Mathar_, Aug 18 2008

%F G.f.: 4*x*(1 - x)/(1 - 4*x). - _Vincenzo Librandi_, May 29 2014

%p seq(coeff(series(4*x*(1-x)/(1-4*x),x,n+1), x, n), n = 1 .. 25); # _Muniru A Asiru_, Oct 21 2018

%t CoefficientList[Series[4 (1 - x)/(1 - 4 x), {x, 0, 40}], x] (* _Vincenzo Librandi_, May 29 2014 *)

%o (Magma) [4] cat [3*4^(n-1): n in [2..30]]; // _Vincenzo Librandi_, May 29 2014

%o (PARI) x='x+O('x^50); Vec(4*x*(1 - x)/(1 - 4*x)) \\ _G. C. Greubel_, Sep 01 2017

%o (GAP) Concatenation([4],List([2..25],n->3*4^(n-1))); # _Muniru A Asiru_, Oct 21 2018

%Y Cf. A000302, A007090, A081604, A110591, A110593.

%K easy,less,nonn

%O 1,1

%A _Jonathan Vos Post_, Jul 29 2005

%E Definition corrected by _R. J. Mathar_, Aug 18 2008