

A107761


Number of permutations of (1,3,5,7,9,...,2n1) where every adjacent pair in the permutation are coprime.


2



1, 2, 6, 24, 72, 480, 3600, 9600, 108000, 1270080, 4795200, 74088000, 768539520, 4759413120, 94182359040, 1893397524480, 11353661706240, 122634632171520, 3104438623534080, 23063946114908160, 664424069072117760
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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, n1}]==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



