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 A206723 #16 Sep 21 2017 12:00:57
%S A206723 7,35,112,315,847,2240,5887,15435,40432,105875,277207,725760,1900087,
%T A206723 4974515,13023472,34095915,89264287,233696960,611826607,1601782875,
%U A206723 4193522032,10978783235,28742827687,75249699840,197006271847,515769115715,1350301075312,3535134110235,9255101255407,24230169656000,63435407712607,166076053481835,434792752732912
%N A206723 a(n) = 7*( ((3 + sqrt(5))/2)^n + ((3 - sqrt(5))/2)^n - 2 ).
%H A206723 Hang Gu and Robert M. Ziff, <a href="http://arxiv.org/abs/1111.5626">Crossing on hyperbolic lattices</a>, arXiv preprint arXiv:1111.5626, 2011
%H A206723 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (4,-4,1).
%F A206723 G.f.: -7*x*(1+x) / ( (x-1)*(x^2-3*x+1) ). - _R. J. Mathar_, Nov 15 2013
%t A206723 LinearRecurrence[{4, -4, 1}, {7, 35, 112}, 33] (* _Jean-François Alcover_, Sep 21 2017 *)
%Y A206723 Equals 7*A004146 for n >= 1.
%K A206723 nonn,easy
%O A206723 1,1
%A A206723 _N. J. A. Sloane_, Feb 11 2012