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 A072561 #11 Feb 16 2021 08:28:48
%S A072561 1,3,4,6,7,9,10,12,13,15,19,21,22,24,25,27,28,30,31,33,37,39,40,42,43,
%T A072561 45,46,48,49,51,55,57,58,60,61,63,64,66,67,69,73,75,76,78,79,81,82,84,
%U A072561 85,87,91,93,94,96,97,99,100,102,103,105,109,111,112,114,115,117,118
%N A072561 Denominators of w(n) equals 3 where w(1)=w(2)=w(3)=1, w(n)=(w(n-1)*w(n-2)+(w(n-1)+w(n-2))/3) / w(n-3).
%C A072561 Denominators of w(k) are 1, 3 or 9.
%H A072561 <a href="/index/Rec#order_11">Index entries for linear recurrences with constant coefficients</a>, signature (1,0,0,0,0,0,0,0,0,1,-1).
%F A072561 lim n -> infinity a(n)/n = 9/5. sequence contains numbers of form (1+18k), (3+18k), (4+18k), (6+18k), (7+18k), (9+18k), (10+18k), (12+18k), (13+18k), (15+18k) k>=0.
%t A072561 LinearRecurrence[{1, 0, 0, 0, 0, 0, 0, 0, 0, 1, -1},{1, 3, 4, 6, 7, 9, 10, 12, 13, 15, 19},67] (* _Ray Chandler_, Aug 25 2015 *)
%Y A072561 Cf. A072560, A072557.
%K A072561 nonn
%O A072561 1,2
%A A072561 _Benoit Cloitre_, Aug 06 2002
%E A072561 Corrected by _Franklin T. Adams-Watters_, Oct 25 2006