A107761 Number of permutations of (1,3,5,7,9,...,2n-1) where every adjacent pair in the permutation are coprime.

1, 2, 6, 24, 72, 480, 3600, 9600, 108000, 1270080, 4795200, 74088000, 768539520, 4759413120, 94182359040, 1893397524480, 11353661706240, 122634632171520, 3104438623534080, 23063946114908160, 664424069072117760

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