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 A133672 #15 Jan 03 2024 08:43:52
%S A133672 1,9,-72,-801,5031,69912,-337599,-6000471,21345048,507383199,
%T A133672 -1221565689,-42319604808,56627216001,3484515205449,-1102289290632,
%U A133672 -283348020932001,-194062588390809,22757127107101272,38476196766756801,-1804851098908446231
%N A133672 a(n) = a(n-1) - 81*a(n-2), a(0)=1, a(1)=9.
%H A133672 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (1,-81).
%F A133672 G.f.: (1+8*x)/(1-x+81*x^2).
%F A133672 a(n) = Sum_{k=0..n} A133607(n,k)*9^k. - _Philippe Deléham_, Dec 30 2007
%t A133672 LinearRecurrence[{1,-81},{1,9},30] (* _Harvey P. Dale_, Aug 01 2016 *)
%K A133672 easy,sign
%O A133672 0,2
%A A133672 _Philippe Deléham_, Dec 29 2007