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A052643
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E.g.f. (1+x-x^2)^2/(1-x)^2.
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0
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1, 4, 12, 36, 168, 960, 6480, 50400, 443520, 4354560, 47174400, 558835200, 7185024000, 99632332800, 1482030950400, 23538138624000, 397533007872000, 7113748561920000, 134449847820288000, 2676192208994304000
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OFFSET
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0,2
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LINKS
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Table of n, a(n) for n=0..19.
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 589
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FORMULA
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E.g.f.: (-x+x^2-1)^2/(-1+x)^2
Recurrence: {a(0)=1, a(1)=4, a(3)=36, a(4)=168, (-n^2-5*n-4)*a(n)+(n+3)*a(n+1)=0, a(2)=12}
(n+3)*n!, n>2.
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MAPLE
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spec := [S, {S=Prod(Union(Z, Sequence(Z)), Union(Z, Sequence(Z)))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
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CROSSREFS
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Sequence in context: A113990 A192010 A056383 * A140123 A164853 A076124
Adjacent sequences: A052640 A052641 A052642 * A052644 A052645 A052646
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KEYWORD
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easy,nonn
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AUTHOR
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encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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STATUS
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approved
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