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 A004643 #27 Sep 08 2022 08:44:33
%S A004643 1,2,10,20,100,200,1000,2000,10000,20000,100000,200000,1000000,
%T A004643 2000000,10000000,20000000,100000000,200000000,1000000000,2000000000,
%U A004643 10000000000,20000000000,100000000000,200000000000
%N A004643 Powers of 2 written in base 4.
%C A004643 Also seems to be numbers k such that the digits of k^3 are cubes. - _Colin Barker_, Apr 21 2017
%H A004643 Vincenzo Librandi, <a href="/A004643/b004643.txt">Table of n, a(n) for n = 0..1000</a>
%H A004643 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (0,10).
%F A004643 From _Chai Wah Wu_, Sep 03 2020: (Start)
%F A004643 a(n) = 10*a(n-2) for n > 1.
%F A004643 G.f.: (-2*x - 1)/(10*x^2 - 1). (End)
%F A004643 a(n) = (3-(-1)^n)/2 * 10^floor(n/2). - _Bernard Schott_, Sep 17 2020
%t A004643 Table[FromDigits[IntegerDigits[2^n, 4]], {n, 0, 40}] (* _Vincenzo Librandi_, Jun 07 2013 *)
%t A004643 Table[{1,2}10^n,{n,0,20}]//Flatten (* _Harvey P. Dale_, Feb 14 2018 *)
%o A004643 (PARI) for(n=0,30, print1(fromdigits(digits(2^n, 4)), ", ")) \\ _G. C. Greubel_, Sep 10 2018
%o A004643 (Magma) [Seqint(Intseq(2^n, 4)): n in [0..30]]; // _G. C. Greubel_, Sep 10 2018
%K A004643 nonn,base,easy
%O A004643 0,2
%A A004643 _N. J. A. Sloane_