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A052520
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Number of pairs of sequences of cardinality at least 2.
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1
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0, 0, 0, 0, 24, 240, 2160, 20160, 201600, 2177280, 25401600, 319334400, 4311014400, 62270208000, 958961203200, 15692092416000, 271996268544000, 4979623993344000, 96035605585920000, 1946321606541312000
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,5
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LINKS
| Vincenzo Librandi, Table of n, a(n) for n = 0..400
Milan Janjic, Enumerative Formulas for Some Functions on Finite Sets
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 87
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FORMULA
| E.g.f.: x^4/(-1+x)^2
Recurrence: {a(1)=0, a(0)=0, a(2)=0, a(3)=0, (n+2-n^2)*a(n)+(n-3)*a(n+1), a(4)=24}
(n-3)*n!, n>2.
a(n)=(n+1)!*(n-3)/(n+1),n>2. [From Gary Detlefs, Oct 02 2011]
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MAPLE
| Pairs spec := [S, {B=Sequence(Z, 2 <= card), S=Prod(B, B)}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
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MATHEMATICA
| Table[Sum[n!, {i, 4, n}], {n, 0, 19}] [From Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Jul 12 2009]
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CROSSREFS
| Sequence in context: A000919 A014340 A052753 * A052724 A000536 A151720
Adjacent sequences: A052517 A052518 A052519 * A052521 A052522 A052523
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KEYWORD
| easy,nonn
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AUTHOR
| encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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