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A291039
a(n) = (1/4)*A291038(n).
2
1, 3, 8, 21, 54, 136, 337, 825, 2000, 4809, 11484, 27264, 64401, 151455, 354808, 828349, 1927986, 4475080, 10361441, 23936565, 55183904, 126983569, 291698040, 669004352, 1532095329, 3503889147, 8003207912, 18258464741, 41608726254, 94722900936, 215428701233
OFFSET
0,2
FORMULA
G.f.: -((-1 + x + x^3)/(-1 + 2 x + x^3)^2).
a(n) = 4*a(n-1) - 4*a(n-2) + 2*a(n-3) - 4*a(n-4) - a(n-6) for n >= 7.
MATHEMATICA
z = 60; s = x/(x - x^3); p = (1 - 2 s)^2;
Drop[CoefficientList[Series[s, {x, 0, z}], x], 1] (* A079978 *)
u = Drop[CoefficientList[Series[1/p, {x, 0, z}], x], 1] (* A291038 *)
u/4 (* A291039 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Clark Kimberling, Sep 14 2017
STATUS
approved