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 A270473 #31 Jul 09 2024 12:45:13
%S A270473 1,4,36,324,2916,26244,236196,2125764,19131876,172186884,1549681956,
%T A270473 13947137604,125524238436,1129718145924,10167463313316,91507169819844,
%U A270473 823564528378596,7412080755407364,66708726798666276,600378541187996484,5403406870691968356,48630661836227715204
%N A270473 Expansion of g.f. (1-5*x)/(1-9*x).
%C A270473 Also squares that can be expressed as the sum of two powers of three (3^x + 3^y), except a(0). - _Karl-Heinz Hofmann_, Sep 03 2022
%H A270473 Colin Barker, <a href="/A270473/b270473.txt">Table of n, a(n) for n = 0..1000</a>
%H A270473 <a href="/index/Rec#order_01">Index entries for linear recurrences with constant coefficients</a>, signature (9).
%F A270473 G.f.: (1-5*x)/(1-9*x).
%F A270473 a(n) = 9*a(n-1) for n>1.
%F A270473 a(n) = 4*9^(n-1) for n>0.
%F A270473 E.g.f.: (4*exp(9*x) + 5)/9. - _Stefano Spezia_, Jul 09 2024
%t A270473 Join[{1},NestList[9#&,4,20]] (* _Harvey P. Dale_, Oct 23 2022 *)
%o A270473 (PARI) Vec((1-5*x)/(1-9*x) + O(x^30))
%Y A270473 Cf. A001019 (powers of 9), A083884 (partial sums).
%Y A270473 Cf. A067403: (1-4*x)/(1-9*x); A102518: (1-6*x)/(1-9*x).
%Y A270473 Cf. A356879, A356880.
%K A270473 nonn,easy
%O A270473 0,2
%A A270473 _Colin Barker_, Mar 17 2016