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A078484
G.f.: -x*(1-2*x+2*x^2)/(2*x^3-4*x^2+4*x-1).
3
0, 1, 2, 6, 18, 52, 148, 420, 1192, 3384, 9608, 27280, 77456, 219920, 624416, 1772896, 5033760, 14292288, 40579904, 115217984, 327136896, 928835456, 2637230208, 7487852800, 21260161280, 60363694336, 171389837824, 486624896512, 1381667623424, 3922950583296
OFFSET
0,3
COMMENTS
Enumerates pop-stacks.
LINKS
M. D. Atkinson and T. Stitt, Restricted permutations and the wreath product, Preprint, 2002.
M. D. Atkinson and T. Stitt, Restricted permutations and the wreath product, Discrete Math., 259 (2002), 19-36.
FORMULA
a(0)=0, a(1)=1, a(2)=2, a(3)=6, a(n) = 4*a(n-1)-4*a(n-2)+2*a(n-3). - Harvey P. Dale, Jul 21 2012
MATHEMATICA
CoefficientList[Series[-x(1-2x+2x^2)/(2x^3-4x^2+4x-1), {x, 0, 30}], x] (* or *) Join[{0}, LinearRecurrence[{4, -4, 2}, {1, 2, 6}, 30]] (* Harvey P. Dale, Jul 21 2012 *)
CROSSREFS
Sequence in context: A128104 A318570 A027059 * A156989 A077935 A077835
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Jan 04 2003
EXTENSIONS
Replaced definition with g.f. given by Atkinson and Stitt (2002). - N. J. A. Sloane, May 24 2016
STATUS
approved