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Search: id:A114938
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| A114938 |
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Number of permutations of the multiset {1,1,2,2,....,n,n} with no two consecutive terms equal. |
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6
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| 0, 2, 30, 864, 39480, 2631600, 241133760, 29083420800, 4467125013120, 851371260364800, 197158144895712000, 54528028997584665600, 17752366094818747392000, 6720318485119046923315200
(list; graph; listen)
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OFFSET
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1,2
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REFERENCES
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R. P. Stanley, Enumerative Combinatorics Volume I, Cambridge University Press, 1997. Chapter 2, Sieve Methods, Example 2.2.3, page 68.
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FORMULA
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a(n)=Sum_{k=0..n}((C(n, k)*(-1)^(n-k)*(n+k)!)/2^k).
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EXAMPLE
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a(2)=2 because there are two permutations of {1,1,2,2} avoiding equal consecutive terms: 1212 and 2121.
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CROSSREFS
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Cf. A114939 = preferred seating arrangements of n couples.
Sequence in context: A020547 A160694 A013525 this_sequence A082653 A140174 A089016
Adjacent sequences: A114935 A114936 A114937 this_sequence A114939 A114940 A114941
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KEYWORD
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nonn
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AUTHOR
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Hugo Pfoertner (hugo(AT)pfoertner.org), Jan 08 2006
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