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Search: id:A077612
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| A077612 |
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Number of adjacent pairs of form (even,even) among all permutations of {1,2,...,n}. |
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+0
4
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| 0, 0, 0, 12, 48, 720, 4320, 60480, 483840, 7257600, 72576000, 1197504000, 14370048000, 261534873600, 3661488230400, 73229764608000, 1171676233728000, 25609494822912000, 460970906812416000
(list; graph; listen)
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OFFSET
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1,4
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LINKS
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Leroy Quet, Home Page (listed in lieu of email address)
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FORMULA
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a(n) = floor(n/2)*floor(n/2-1)*(n-1)!. Proof: There are floor(n/2)*floor(n/2-1) pairs (r, s) with r and s even and distinct. For each pair, there are n-1 places it can occur in a permutation and (n-2)! possible arrangements of the other numbers.
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CROSSREFS
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Cf. A077611, A077613.
Sequence in context: A052601 A003498 A002899 this_sequence A041272 A022282 A009959
Adjacent sequences: A077609 A077610 A077611 this_sequence A077613 A077614 A077615
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KEYWORD
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nonn
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AUTHOR
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Leroy Quet Frank Ruskey (fruskey(AT)cs.uvic.ca) and Dean Hickerson (dean.hickerson(AT)yahoo.com), Nov 11 2002
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