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 A253101 #34 Jan 09 2019 03:54:32
%S A253101 1,6,24,96,372,1416,5340,20040,75012,280392,1047324,3910440,14597508,
%T A253101 54485736,203357724,758969736,2832570372,10571410056,39453266460,
%U A253101 147242049000,549515715972,2050822387752,7653776980764,28564291826760,106603402909188,397849344975816,1484794027325724,5541326864990376
%N A253101 a(n) = A253100(2^n-1).
%H A253101 Colin Barker, <a href="/A253101/b253101.txt">Table of n, a(n) for n = 0..1000</a>
%H A253101 Shalosh B. Ekhad, N. J. A. Sloane, and Doron Zeilberger, <a href="http://arxiv.org/abs/1503.01796">A Meta-Algorithm for Creating Fast Algorithms for Counting ON Cells in Odd-Rule Cellular Automata</a>, arXiv:1503.01796 [math.CO], 2015; see also the <a href="http://www.math.rutgers.edu/~zeilberg/mamarim/mamarimhtml/CAcount.html">Accompanying Maple Package</a>.
%H A253101 Shalosh B. Ekhad, N. J. A. Sloane, and Doron Zeilberger, <a href="http://arxiv.org/abs/1503.04249">Odd-Rule Cellular Automata on the Square Grid</a>, arXiv:1503.04249 [math.CO], 2015.
%H A253101 N. J. A. Sloane, On the No. of ON Cells in Cellular Automata, Video of talk in Doron Zeilberger's Experimental Math Seminar at Rutgers University, Feb. 05 2015: <a href="https://vimeo.com/119073818">Part 1</a>, <a href="https://vimeo.com/119073819">Part 2</a>
%H A253101 N. J. A. Sloane, <a href="http://arxiv.org/abs/1503.01168">On the Number of ON Cells in Cellular Automata</a>, arXiv:1503.01168 [math.CO], 2015.
%H A253101 <a href="/index/Ce#cell">Index entries for sequences related to cellular automata</a>
%H A253101 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (6,-9,2).
%F A253101 G.f.: (1-3*x^2+4*x^3) / ((1-2*x)*(1-4*x+x^2)).
%F A253101 From _Colin Barker_, Feb 04 2017: (Start)
%F A253101 a(n) = -2^n + 2*(2-sqrt(3))^n + 2*(2+sqrt(3))^n for n>0.
%F A253101 a(n) = 6*a(n-1) - 9*a(n-2) + 2*a(n-3) for n>3.
%F A253101 (End)
%t A253101 Join[{1}, LinearRecurrence[{6, -9, 2}, {6, 24, 96}, 30]] (* _Jean-François Alcover_, Jan 09 2019 *)
%o A253101 (PARI) Vec((1-3*x^2+4*x^3) / ((1-2*x)*(1-4*x+x^2)) + O(x^30)) \\ _Colin Barker_, Feb 04 2017
%Y A253101 Cf. A253100.
%K A253101 nonn,easy
%O A253101 0,2
%A A253101 _N. J. A. Sloane_ and _Doron Zeilberger_, Feb 19 2015
%E A253101 Typo in the definition corrected by _Antti Karttunen_, Nov 18 2016