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 A175805 #34 Apr 22 2025 06:32:00
%S A175805 21,42,84,168,336,672,1344,2688,5376,10752,21504,43008,86016,172032,
%T A175805 344064,688128,1376256,2752512,5505024,11010048,22020096,44040192,
%U A175805 88080384,176160768,352321536,704643072,1409286144,2818572288,5637144576,11274289152,22548578304,45097156608
%N A175805 a(n) = 21*2^n.
%C A175805 The squares of these terms appear in A159256. - _Elmo R. Oliveira_, Sep 15 2024
%H A175805 Vincenzo Librandi, <a href="/A175805/b175805.txt">Table of n, a(n) for n = 0..1000</a>
%H A175805 <a href="/index/Rec#order_01">Index entries for linear recurrences with constant coefficients</a>, signature (2).
%F A175805 G.f.: 21/(1-2*x).
%F A175805 a(n) = 2*a(n-1). - _Vincenzo Librandi_, Jul 08 2016
%F A175805 From _Elmo R. Oliveira_, Sep 14 2024: (Start)
%F A175805 E.g.f.: 21*exp(2*x).
%F A175805 a(n) = 21*A000079(n). (End)
%t A175805 Table[21 2^n, {n, 0, 40}] (* _Vincenzo Librandi_, Jul 08 2016 *)
%o A175805 (Magma) [21*2^n: n in [0..100]];
%o A175805 (PARI) a(n)=21<<n \\ _Charles R Greathouse IV_, Oct 07 2015
%Y A175805 Cf. A000079, A159256.
%K A175805 nonn,easy
%O A175805 0,1
%A A175805 _Vincenzo Librandi_, Dec 05 2010