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 A004658 #15 Sep 08 2022 08:44:33
%S A004658 1,3,21,123,1101,3303,23121,202023,1212201,10303203,32122221,
%T A004658 223033323,2001233301,12011033103,102033231321,312233021223,
%U A004658 2210031131001,13230220113003,113011321011021,1011101223033123
%N A004658 Powers of 3 written in base 4.
%H A004658 Vincenzo Librandi, <a href="/A004658/b004658.txt">Table of n, a(n) for n = 0..1000</a>
%t A004658 Table[FromDigits[IntegerDigits[3^n, 4]], {n, 0, 40}] (* _Vincenzo Librandi_, Jun 07 2013 *)
%o A004658 (PARI) a(n,b=4,m=3)=fromdigits(digits(m^n,b)) \\ _M. F. Hasler_, Jun 22 2018
%o A004658 (Magma) [Seqint(Intseq(3^n, 4)): n in [0..30]]; // _G. C. Greubel_, Sep 10 2018
%Y A004658 Cf. A000244, A004656, A004659, ... : powers of 3 written in base 10, 2, 5, ...
%Y A004658 Cf. A000079, A004643, ..., A004655: powers of 2 written in base 10, 4, 5, ..., 16
%K A004658 nonn,base,easy
%O A004658 0,2
%A A004658 _N. J. A. Sloane_