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 A004669 #21 Sep 04 2020 06:11:53
%S A004669 1,3,9,10,30,90,100,300,900,1000,3000,9000,10000,30000,90000,100000,
%T A004669 300000,900000,1000000,3000000,9000000,10000000,30000000,90000000,
%U A004669 100000000,300000000,900000000,1000000000
%N A004669 Powers of 3 written in base 27.
%C A004669 Similar to powers of 2 in base 8 (A004647) or 16 (A004655). - _M. F. Hasler_, Jun 22 2018
%H A004669 G. C. Greubel, <a href="/A004669/b004669.txt">Table of n, a(n) for n = 0..1000</a>
%H A004669 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (0,0,10).
%F A004669 a(n) = 3^(n mod 3)*10^floor(n/3). - _M. F. Hasler_, Jun 22 2018
%F A004669 From _Chai Wah Wu_, Sep 03 2020: (Start)
%F A004669 a(n) = 10*a(n-3) for n > 2.
%F A004669 G.f.: (-9*x^2 - 3*x - 1)/(10*x^3 - 1). (End)
%t A004669 Table[FromDigits[IntegerDigits[3^n, 27]], {n, 0, 100}] (* _G. C. Greubel_, Oct 12 2018 *)
%o A004669 (PARI) apply( a(n)=3^(n%3)*10^(n\3), [0..20]) \\ _M. F. Hasler_, Jun 22 2018
%Y A004669 Cf. A000079, A004643, ..., A004655: powers of 2 written in base 10, 4, 5, ..., 16.
%Y A004669 Cf. A000244, A004656, A004658, A004659, ..., A004668: powers of 3 in base 10, 2, 4, 5, ..., 26.
%K A004669 nonn,easy,base
%O A004669 0,2
%A A004669 _N. J. A. Sloane_