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Search: id:A052747
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| A052747 |
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a(0) = a(1) = a(2) = 0; a(n) = n!/(n-2) (n > 2). |
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+0
4
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| 0, 0, 0, 6, 12, 40, 180, 1008, 6720, 51840, 453600, 4435200, 47900160, 566092800, 7264857600, 100590336000, 1494484992000, 23712495206400, 400148356608000, 7155594141696000, 135161222676480000
(list; graph; listen)
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OFFSET
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0,4
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COMMENT
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A simple grammar.
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LINKS
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INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 703
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FORMULA
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E.g.f.: ln(-1/(-1+x))*x^2
Recurrence: {a(0)=0, a(1)=0, a(2)=0, a(3)=6, (n+2-n^2)*a(n)+(n-1)*a(n+1)}
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MAPLE
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spec := [S, {B=Cycle(Z), S=Prod(Z, Z, B)}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
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CROSSREFS
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Contribution from Johannes W. Meijer (meijgia(AT)hotmail.com), Jul 21 2009: (Start)
Equals the square root of the second right hand column of A162990 for n=>3.
(End)
Sequence in context: A096377 A026083 A128453 this_sequence A007121 A152786 A048069
Adjacent sequences: A052744 A052745 A052746 this_sequence A052748 A052749 A052750
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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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EXTENSIONS
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Better description from Olivier Gerard (olivier.gerard(AT)gmail.com), Jun 13 2001
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