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A305650
a(n) = -1/3 * (u^n-1)*(v^n-1) with u = 1+sqrt(3), v = 1-sqrt(3).
1
1, 1, 9, 13, 61, 117, 421, 949, 2997, 7381, 21781, 56277, 160213, 424789, 1186389, 3189589, 8817493, 23883093, 65663317, 178568533, 489512277, 1334064469, 3651347797, 9962435925, 27244344661, 74380006741, 203315811669, 555257419093, 1517414896981, 4144807761237
OFFSET
1,3
FORMULA
a(n) = a(n-1) + 6*a(n-2) - 2*a(n-3) - 4*a(n-4) for n > 4.
G.f.: x*(1 + 2*x^2) / ((1 - x)*(1 + 2*x)*(1 - 2*x - 2*x^2)). - Colin Barker, Jun 07 2018
MATHEMATICA
LinearRecurrence[{1, 6, -2, -4}, {1, 1, 9, 13}, 30] (* Harvey P. Dale, Jun 02 2019 *)
PROG
(PARI) Vec(x*(1 + 2*x^2) / ((1 - x)*(1 + 2*x)*(1 - 2*x - 2*x^2)) + O(x^40)) \\ Colin Barker, Jun 07 2018
CROSSREFS
Cf. A129744.
Sequence in context: A365239 A271006 A271052 * A099580 A258286 A166662
KEYWORD
nonn,easy
AUTHOR
Seiichi Manyama, Jun 07 2018
STATUS
approved