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Search: id:A107761
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| A107761 |
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Number of permutations of (1,3,5,7,9,...,2n-1) where every adjacent pair in the permutation are coprime. |
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3
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| 1, 2, 6, 24, 72, 480, 3600, 9600, 108000, 1270080, 4795200, 74088000, 768539520, 4759413120, 94182359040, 1893397524480, 11353661706240, 122634632171520, 3104438623534080, 23063946114908160, 664424069072117760
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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Odd analogue of A076220.
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REFERENCES
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a(1)-a(9) computed by Zak Seidov.
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EXAMPLE
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For example, if n = 5, the permutation (5,3,7,9,1) is counted, but (5,3,9,1,7) is not counted because 3 and 9 are adjacent.
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MATHEMATICA
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With[{n=9}, per=Permutations[Range[1, 2 n -1, 2]]; Select[per, Times @@ Table[GCD @@Partition[ #, 2, 1][[i]], {i, n-1}]==1&]//Length] (Seidov)
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CROSSREFS
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Cf. A076220, A086595, A102381, A107762, A107763.
Sequence in context: A027562 A096259 A087645 this_sequence A002526 A117665 A068777
Adjacent sequences: A107758 A107759 A107760 this_sequence A107762 A107763 A107764
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KEYWORD
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nonn
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AUTHOR
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Ray Chandler (rayjchandler(AT)sbcglobal.net), following a suggestion of Leroy Quet, Jun 11 2005
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EXTENSIONS
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More terms from Max Alekseyev (maxal(AT)cs.ucsd.edu), Jun 11 2005
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