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 A109545 #13 Jan 10 2019 22:54:42
%S A109545 1,1,2,6,15,38,97,247,629,1602,4080,10391,26464,67399,171653,437169,
%T A109545 1113390,2835602,7221763,18392518,46842401,119299083,303833085,
%U A109545 773807654,1970747476,5019135691,12782826512,32555536191,82913034585
%N A109545 a(n) = 2*a(n-1) + a(n-2) + a(n-3).
%H A109545 David Garth and Kevin G. Hare, <a href="http://dx.doi.org/10.1016/j.jnt.2006.02.003">Comments on the spectra of Pisot numbers</a>, J. Number Theory 121 (2006), 187-203.
%H A109545 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (2, 1, 1).
%F A109545 lim_{n-> infinity} a(n)/a(n-1)= 2.54682...
%F A109545 G.f.: (1-x-x^2)/(1-2*x-x^2-x^3). [Sep 28 2009]
%F A109545 a(n) = A077939(n)-A116413(n-1).
%F A109545 G.f.: (-1+x+x^2)/(-1+2*x+x^2+x^3). a(n) = A077997(n)-A077939(n-2). [From _R. J. Mathar_, Sep 27 2009]
%t A109545 a = 2; b = -1; M = {{0, 1, 0, 0, 0}, { a - 2, a - 2, a - 2 - b, a - 2 - b, 0}, {1, 1, 1, 1, 0}, {0, 1, 1, 0, 0}, {0, 0, 0, 1, 1}} v[1] = {1, 1, 1, 1, 1} v[n_] := v[n] = M.v[n - 1] a0 = Table[Abs[v[n][[1]]], {n, 1, 50}]
%t A109545 LinearRecurrence[{2,1,1},{1,1,2},30] (* _Harvey P. Dale_, Aug 05 2015 *)
%t A109545 Lucas := 1 + x (1 + 2 x)/(1 - x - x^2); (* InvertTransform defined in A052987 *)
%t A109545 InvertTransform[Lucas, 28] (* _Peter Luschny_, Jan 10 2019 *)
%K A109545 nonn,easy
%O A109545 0,3
%A A109545 _Roger L. Bagula_, Jun 20 2005
%E A109545 Definition replaced by recurrence by the Associate Editors of the OEIS, Sep 28 2009