A107761 - OEIS

%I #18 Sep 30 2015 10:47:41

%S 1,2,6,24,72,480,3600,9600,108000,1270080,4795200,74088000,768539520,

%T 4759413120,94182359040,1893397524480,11353661706240,122634632171520,

%U 3104438623534080,23063946114908160,664424069072117760

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

%C Odd analog of A076220.

%e 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.

%t 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)

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

%K nonn

%O 1,2

%A _Ray Chandler_, following a suggestion of _Leroy Quet_, Jun 11 2005

%E a(1)-a(9) computed by _Zak Seidov_.

%E More terms from _Max Alekseyev_, Jun 11 2005