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%I #12 Oct 12 2015 09:02:02
%S 1,8,32,120,448,1672,6240,23288,86912,324360,1210528,4517752,16860480,
%T 62924168,234836192,876420600,3270846208,12206964232,45557010720,
%U 170021078648,634527303872,2368088136840,8837825243488,32983212837112,123095026104960,459396891582728,1714492540225952,6398573269321080
%N G.f.: (1+4*x+x^2)/(1-4*x+x^2).
%H Colin Barker, <a href="/A234272/b234272.txt">Table of n, a(n) for n = 0..1000</a>
%H J. Choi, N. Pippenger, <a href="http://arxiv.org/abs/1310.1357">Counting the Angels and Devils in Escher's Circle Limit IV</a>, arXiv preprint arXiv:1310.1357, 2013
%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (4,-1).
%F a(n) = 8*A001353(n), n>0. - _R. J. Mathar_, Jan 21 2014
%F From _Colin Barker_, Oct 12 2015: (Start)
%F a(n) = 4*a(n-1)-a(n-2) for n>2.
%F a(n) = 4*(-(2-sqrt(3))^n+(2+sqrt(3))^n)/sqrt(3) for n>1.
%F (End)
%t CoefficientList[Series[(1+4x+x^2)/(1-4x+x^2),{x,0,30}],x] (* or *) LinearRecurrence[{4,-1},{1,8,32},30] (* _Harvey P. Dale_, Jul 12 2015 *)
%o (PARI) Vec((1+4*x+x^2)/(1-4*x+x^2) + O(x^40)) \\ _Colin Barker_, Oct 12 2015
%K nonn,easy
%O 0,2
%A _N. J. A. Sloane_, Dec 24 2013
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