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A291384
a(n) = (1/2)*A291383(n).
2
1, 4, 14, 49, 172, 604, 2120, 7442, 26124, 91704, 321912, 1130020, 3966752, 13924640, 48880192, 171585992, 602324816, 2114363648, 7422130912, 26054187664, 91459003200, 321052008000, 1127001041280, 3956154502688, 13887439208896, 48749604609664
OFFSET
0,2
FORMULA
G.f.: -(((1 + x) (1 + x + x^2))/(-1 + 2 x + 4 x^2 + 4 x^3 + 2 x^4))
a(n) = 2*a(n-1) + 4*a(n-2) + 4*a(n-3) + 2*a(n-4) for n >= 5.
MATHEMATICA
z = 60; s = x + x^2; p = 1 - 2 s - 2 s^2;
Drop[CoefficientList[Series[s, {x, 0, z}], x], 1] (* A019590 *)
u = Drop[CoefficientList[Series[1/p, {x, 0, z}], x], 1] (* A291383 *)
u / 2 (* A291384 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Clark Kimberling, Sep 04 2017
STATUS
approved