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 A209239 #31 Oct 13 2022 02:48:31
%S A209239 1,3,9,27,80,238,708,2106,6264,18632,55420,164844,490320,1458432,
%T A209239 4338032,12903256,38380080,114159600,339561936,1010009744,3004222720,
%U A209239 8935908000,26579404800,79059090528,235157252096,699463310848
%N A209239 Number of length n words on {0,1,2} with no four consecutive 0's.
%D A209239 R. Sedgewick and P. Flajolet, Analysis of Algorithms, Addison and Wesley, 1996, page 377.
%H A209239 D. Birmajer, J. B. Gil, and M. D. Weiner, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL19/Gil/gil6.html">On the Enumeration of Restricted Words over a Finite Alphabet</a>, J. Int. Seq. 19 (2016) # 16.1.3, Example 7.
%H A209239 Milan Janjic, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL18/Janjic/janjic63.html">On Linear Recurrence Equations Arising from Compositions of Positive Integers</a>, J. Int. Seq. 18 (2015) # 15.4.7.
%H A209239 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (2,2,2,2).
%F A209239 O.g.f.: (1 - x^4)/(1 - 3*x+ 2*x^5) = (1+x)*(1+x^2)/(1-2*x-2*x^2-2*x^3-2*x^4).
%F A209239 a(n) = A160175(n) + A160175(n-1) + A160175(n-2) + A160175(n-3). - _R. J. Mathar_, Aug 04 2019
%F A209239 a(n) = 2*(a(n-1) + a(n-2) + a(n-3) + a(n-4)) for n>=4, with a(0) = 1, a(1) = 3, a(2) = 9, a(3) = 27. - _Taras Goy_, Aug 04 2019
%t A209239 nn=25; CoefficientList[Series[(1-x^4)/(1-3x+2x^5), {x,0,nn}], x]
%t A209239 LinearRecurrence[{2,2,2,2},{1,3,9,27},40] (* _Harvey P. Dale_, Sep 13 2018 *)
%Y A209239 Cf. A000079, A028859, A119826.
%K A209239 nonn,easy
%O A209239 0,2
%A A209239 _Geoffrey Critzer_, Jan 13 2013