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A052811
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A simple grammar: sequences of pairs of cycles.
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2
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1, 0, 2, 6, 46, 340, 3308, 36288, 460752, 6551424, 103685232, 1803956880, 34247483664, 704301934752, 15598712592864, 370149922235520, 9369093828260736, 251968378971718656, 7174943434198029312
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,3
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COMMENTS
| Stirling transform of (-1)^n*a(n)=[0,2,-6,46,-340,...] is A005359(n)=[0,2,0,24,0,...]. - Michael Somos Mar 04 2004
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LINKS
| INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 775
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FORMULA
| a(n) = (-1)^n*Sum_{k=0..floor(n/2)} Stirling1(n, 2*k)*(2*k)!. - Vladeta Jovovic (vladeta(AT)eunet.rs), Sep 22 2003
E.g.f.: 1/(1-log(1-x)^2).
a(n) = D^n(1/(1-x^2)) evaluated at x = 0, where D is the operator exp(x)*d/dx. Cf. A006252. - Peter Bala, Nov 25 2011
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MAPLE
| spec := [S, {B=Cycle(Z), C=Prod(B, B), S=Sequence(C)}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
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PROG
| (PARI) a(n)=if(n<0, 0, n!*polcoeff(1/(1-log(1-x)^2), n))
(Maxima) makelist((-1)^n*sum(stirling1(n, 2*k)*(2*k)!, k, 0, floor(n/2)), n, 0, 18); [Bruno Berselli, May 25 2011]
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CROSSREFS
| Sequence in context: A171690 A136557 A092662 * A078603 A001587 A078537
Adjacent sequences: A052808 A052809 A052810 * A052812 A052813 A052814
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KEYWORD
| easy,nonn
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AUTHOR
| encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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