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

Author

Ray Chandler, following a suggestion of Leroy Quet, Jun 11 2005

Keywords

• nonn

Comments

Odd analog of A076220.

Examples

			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.
		

Crossrefs

Cf. A076220, A086595, A102381, A107762, A107763.

Programs

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)

Extensions

a(1)-a(9) computed by Zak Seidov.

More terms from Max Alekseyev, Jun 11 2005