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A234272 G.f.: (1+4*x+x^2)/(1-4*x+x^2). 1
1, 8, 32, 120, 448, 1672, 6240, 23288, 86912, 324360, 1210528, 4517752, 16860480, 62924168, 234836192, 876420600, 3270846208, 12206964232, 45557010720, 170021078648, 634527303872, 2368088136840, 8837825243488, 32983212837112, 123095026104960, 459396891582728, 1714492540225952, 6398573269321080 (list; graph; refs; listen; history; text; internal format)
OFFSET
0,2
LINKS
J. Choi, N. Pippenger, Counting the Angels and Devils in Escher's Circle Limit IV, arXiv preprint arXiv:1310.1357, 2013
FORMULA
a(n) = 8*A001353(n), n>0. - R. J. Mathar, Jan 21 2014
From Colin Barker, Oct 12 2015: (Start)
a(n) = 4*a(n-1)-a(n-2) for n>2.
a(n) = 4*(-(2-sqrt(3))^n+(2+sqrt(3))^n)/sqrt(3) for n>1.
(End)
MATHEMATICA
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 *)
PROG
(PARI) Vec((1+4*x+x^2)/(1-4*x+x^2) + O(x^40)) \\ Colin Barker, Oct 12 2015
CROSSREFS
Sequence in context: A003201 A318944 A196097 * A269077 A183915 A357789
KEYWORD
nonn,easy
AUTHOR
N. J. A. Sloane, Dec 24 2013
STATUS
approved

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Last modified April 22 02:54 EDT 2024. Contains 371887 sequences. (Running on oeis4.)