Permutation of odds: Coprime Adjacents

[zak seidov]

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]
=> 108000
And for n from 1 to 9 we have (not in OEIS?) 
1, 2, 6, 24, 72, 480, 3600, 9600,108000
For n=10 one needs more CP time ;=) 
Zak


--- Leroy Quet <qq-quet at mindspring.com> wrote:

> Could someone please calculate/submit the sequence
> (or is it already in 
> the OEIS?) where the nth term is the number of
> permutations of 
> (1,3,5,7,9,...,2n-1) where every adjacent pair in
> the permutations is 
> coprime.
> 
> 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.
> 
> thanks,
> Leroy Quet
>  
> 



		
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