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

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