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 A107761 Number of permutations of (1,3,5,7,9,...,2n-1) where every adjacent pair in the permutation are coprime. 2
 1, 2, 6, 24, 72, 480, 3600, 9600, 108000, 1270080, 4795200, 74088000, 768539520, 4759413120, 94182359040, 1893397524480, 11353661706240, 122634632171520, 3104438623534080, 23063946114908160, 664424069072117760 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Odd analog of A076220. LINKS EXAMPLE 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. MATHEMATICA 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) CROSSREFS Cf. A076220, A086595, A102381, A107762, A107763. Sequence in context: A216158 A178847 A173844 * A147943 A147934 A147925 Adjacent sequences:  A107758 A107759 A107760 * A107762 A107763 A107764 KEYWORD nonn AUTHOR Ray Chandler, following a suggestion of Leroy Quet, Jun 11 2005 EXTENSIONS a(1)-a(9) computed by Zak Seidov. More terms from Max Alekseyev, Jun 11 2005 STATUS approved

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Last modified February 23 00:28 EST 2020. Contains 332157 sequences. (Running on oeis4.)