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Search: id:A052609
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| 0, 0, 4, 24, 144, 960, 7200, 60480, 564480, 5806080, 65318400, 798336000, 10538035200, 149448499200, 2266635571200, 36614882304000, 627683696640000, 11381997699072000, 217680705994752000
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OFFSET
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0,3
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COMMENT
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Number of permutations of {1,2,...,n+2} such that there are exactly two entries between the entries 1 and 2. Example: a(2)=4 because we have 1342, 1432, 2341 and 2431. - Emeric Deutsch (deutsch(AT)duke.poly.edu), Apr 06 2008
a(n)=A138770(n+2). - Emeric Deutsch (deutsch(AT)duke.poly.edu), Apr 06 2008
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LINKS
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INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 554
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FORMULA
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E.g.f.: 2*x^2/(-1+x)^2
Recurrence: {a(1)=0, a(0)=0, a(2)=4, (-n^2-n)*a(n)+(n-1)*a(n+1)}
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MAPLE
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spec := [S, {S=Prod(Z, Sequence(Z), Sequence(Z), Union(Z, Z))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
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CROSSREFS
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Cf. A138770.
Sequence in context: A121102 A067411 A045915 this_sequence A077613 A072949 A104531
Adjacent sequences: A052606 A052607 A052608 this_sequence A052610 A052611 A052612
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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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