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 A108136 #16 Jul 31 2015 17:54:34
%S A108136 1,1,1,6,21,76,276,1001,3631,13171,47776,173301,628626,2280256,
%T A108136 8271321,30003101,108832201,394774126,1431989881,5194350096,
%U A108136 18841804176,68346102601,247916266251,899282808131,3262027059496,11832563061001
%N A108136 a(1)=1; a(2)=1; a(3)=1; a(n) = 3*a(n-1) + 2*a(n-2) + a(n-3).
%C A108136 Length of steps in the 3-symbol substitution 1->{2}, 2->{3}, 3->{1,2,2,3,3,3} with characteristic polynomial: x^3 - 3*x^2 - 2*x - 1.
%C A108136 No term is divisible by 5. - _Vladimir Joseph Stephan Orlovsky_, Mar 24 2011
%H A108136 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3, 2, 1).
%F A108136 From _R. J. Mathar_, Oct 14 2008: (Start)
%F A108136 G.f.: x*(1 - 2*x - 4*x^2)/(1 - 3*x - 2*x^2 - x^3).
%F A108136 a(n) = A108153(n) - 2*A108153(n-1) - 4*A108153(n-2). (End)
%t A108136 LinearRecurrence[{3, 2, 1}, {1, 1, 1}, 30]
%K A108136 nonn,easy
%O A108136 1,4
%A A108136 _Roger L. Bagula_, Jun 05 2005