# Permutation of odds: Coprime Adjacents

Ray Chandler Ray.Chandler at comcast.net
Sat Jun 11 05:10:26 CEST 2005

```This sequence is the "odd" analog of A076220 which is related to A086595 and A102381.

Odd analog of A076220 = 1, 2, 6, 24, 72, 480, 3600, 9600, 108000, 1270080,...

Odd analog of A086595 = 1, 2, 6, 24, 60, 432, 3360, 6912, 86400, 1080000,...

Odd analog of A102381 = 1, 1, 2, 6, 12, 72, 480, 864, 9600, 108000,...

I can find none of these sequences in the OEIS.

Ray Chandler

-----Original Message-----
From: zak seidov [mailto:zakseidov at yahoo.com]
Sent: Friday, June 10, 2005 2:58 PM
To: Leroy Quet
Cc: seqfan at ext.jussieu.fr
Subject: Re: Permutation of odds: Coprime Adjacents

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:

> 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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