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A052791
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3^(n-3)*n*(n-1)*(n-2).
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1
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0, 0, 0, 6, 72, 540, 3240, 17010, 81648, 367416, 1574640, 6495390, 25981560, 101328084, 386889048, 1450833930, 5356925280, 19514513520, 70252248672, 250273635894, 883318714920
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,4
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COMMENTS
| The number of surjective functions f:{1,2,...,n}->{1,2,3} with a designated pre-image of 1,2,and 3.
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LINKS
| INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 748
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FORMULA
| E.g.f.: x^3*exp(x)^3
Recurrence: {a(1)=0, a(2)=0, a(3)=6, (-3*n-3)*a(n)+(-2+n)*a(n+1)}.
a(n) = n!*sum(i+j+k=n, ijk/(i!j!k!)) - Benoit Cloitre (benoit7848c(AT)orange.fr), Nov 11 2004
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MAPLE
| spec := [S, {B=Set(Z), S=Prod(Z, Z, Z, B, B, B)}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
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MATHEMATICA
| Range[0, 20]! CoefficientList[Series[(x Exp[x])^2, {x, 0, 20}], x]
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CROSSREFS
| Cf. A001815.
Sequence in context: A052615 A192990 A177468 * A129532 A151719 A118313
Adjacent sequences: A052788 A052789 A052790 * A052792 A052793 A052794
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KEYWORD
| easy,nonn
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AUTHOR
| encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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EXTENSIONS
| Edited by N. J. A. Sloane, Dec 24 2010
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