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a(n) = 21*2^n.
20

%I #31 Sep 17 2024 15:42:07

%S 21,42,84,168,336,672,1344,2688,5376,10752,21504,43008,86016,172032,

%T 344064,688128,1376256,2752512,5505024,11010048,22020096,44040192,

%U 88080384,176160768,352321536,704643072,1409286144,2818572288,5637144576,11274289152,22548578304,45097156608

%N a(n) = 21*2^n.

%C The squares of these terms appear in A159256. - _Elmo R. Oliveira_, Sep 15 2024

%H Vincenzo Librandi, <a href="/A175805/b175805.txt">Table of n, a(n) for n = 0..1000</a>

%H <a href="/index/Rec#order_01">Index entries for linear recurrences with constant coefficients</a>, signature (2).

%F G.f.: 21/(1-2*x).

%F a(n) = 2*a(n-1). - _Vincenzo Librandi_, Jul 08 2016

%F From _Elmo R. Oliveira_, Sep 14 2024: (Start)

%F E.g.f.: 21*exp(2*x).

%F a(n) = 21*A000079(n). (End)

%t Table[21 2^n, {n, 0, 40}] (* _Vincenzo Librandi_, Jul 08 2016 *)

%o (Magma) [21*2^n: n in [0..100]];

%o (PARI) a(n)=21<<n \\ _Charles R Greathouse IV_, Oct 07 2015

%Y Cf. A000079, A159256.

%K nonn,easy

%O 0,1

%A _Vincenzo Librandi_, Dec 05 2010