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 A133681 #8 Jan 02 2024 08:56:38
%S A133681 1,9,162,2268,34992,519048,7820712,117100728,1757770632,26359001208,
%T A133681 395430367752,5931183418488,88969383864072,1334530962440568,
%U A133681 20018023209731592,300269995507235448,4504052048440962312
%N A133681 a(n)=9*a(n-1)+90*a(n-2) for n>=3, a(0)=1, a(1)=9, a(2)=162 .
%F A133681 G.f.: (1-9*x^2)/(1-9*x-90*x^2) . a(n) = Sum_{k, 0<=k<=n}A122950(n,k)*9^k .
%K A133681 easy,nonn
%O A133681 0,2
%A A133681 _Philippe Deléham_, Dec 31 2007