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Search: id:A052520
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| A052520 |
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Number of pairs of sequences of cardinality at least 2. |
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+0
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; listen)
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OFFSET
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0,5
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LINKS
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Milan Janjic, Enumerative Formulas for Some Functions on Finite Sets
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 87
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FORMULA
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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.
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MAPLE
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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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CROSSREFS
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Sequence in context: A000919 A014340 A052753 this_sequence A052724 A000536 A052652
Adjacent sequences: A052517 A052518 A052519 this_sequence A052521 A052522 A052523
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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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