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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

Table of n, a(n) for n=1..21.

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 September 26 10:59 EDT 2017. Contains 292518 sequences.