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A052636
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E.g.f. (2-x-2x^2)/((1-x)(1-2x^2)).
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0
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2, 1, 6, 6, 120, 120, 6480, 5040, 685440, 362880, 119750400, 39916800, 31135104000, 6227020800, 11245999564800, 1307674368000, 5377157001216000, 355687428096000, 3284417711038464000
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,1
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LINKS
| INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 582
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FORMULA
| E.g.f.: -(-2+x+2*x^2)/(-1+2*x^2)/(-1+x)
Recurrence: {a(1)=1, a(2)=6, a(0)=2, (12+2*n^3+12*n^2+22*n)*a(n) +(-2*n^2-10*n-12)*a(n+1) +(-n-3)*a(n+2) +a(n+3)=0}
(1+Sum(1/2*_alpha^(-n), _alpha=RootOf(-1+2*_Z^2)))*n!
n!*[2^(n/2)+1] if n is even, n! otherwise.
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MAPLE
| spec := [S, {S=Union(Sequence(Prod(Z, Union(Z, Z))), Sequence(Z))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
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CROSSREFS
| Sequence in context: A113979 A053442 A019082 * A172430 A084312 A066752
Adjacent sequences: A052633 A052634 A052635 * A052637 A052638 A052639
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KEYWORD
| easy,nonn
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AUTHOR
| encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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