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 A018091 #19 Apr 22 2025 12:19:33
%S A018091 1,23,370,5150,66451,819413,9810340,115087100,1330524901,15216894803,
%T A018091 172615989910,1945847389850,21827965327351,243920416546193,
%U A018091 2717439768299080,30200674142257400,334986945826585801,3709872176577373583,41033735895102897850,453397372422060233750
%N A018091 Expansion of 1/((1-3*x)*(1-9*x)*(1-11*x)).
%H A018091 Vincenzo Librandi, <a href="/A018091/b018091.txt">Table of n, a(n) for n = 0..200</a>
%H A018091 <a href="/index/Rec">Index entries for linear recurrences with constant coefficients</a>, signature (23,-159,297).
%F A018091 a(0)=1, a(1)=23, a(2)=370; for n>2, a(n) = 23*a(n-1) -159*a(n-2) +297*a(n-3). - _Vincenzo Librandi_, Jul 02 2013
%F A018091 a(n) = 20*a(n-1) -99*a(n-2) +3^n. - _Vincenzo Librandi_, Jul 02 2013
%F A018091 a(n) = (3*11^(n+2) - 4*9^(n+2) + 3^(n+2))/48. - _Yahia Kahloune_, Jul 06 2013
%t A018091 CoefficientList[Series[1 / ((1 - 3 x) (1 - 9 x) (1 - 11 x)), {x, 0, 20}], x] (* _Vincenzo Librandi_, Jul 02 2013 *)
%o A018091 (Magma) m:=20; R<x>:=PowerSeriesRing(Integers(), m); Coefficients(R!(1/((1-3*x)*(1-9*x)*(1-11*x)))); // _Vincenzo Librandi_, Jul 02 2013
%o A018091 (Magma) I:=[1, 23, 370]; [n le 3 select I[n] else 23*Self(n-1)-159*Self(n-2)+297*Self(n-3): n in [1..20]]; // _Vincenzo Librandi_, Jul 02 2013
%K A018091 nonn,easy
%O A018091 0,2
%A A018091 _N. J. A. Sloane_