A137749 Number of circular permutations of the multiset {1,1,2,2,...,2n,2n} (up to rotations) with even distances between equal elements.

1, 30, 13560, 27785520, 162030637440, 2156625389318400, 56857271240920550400, 2686506065987036477184000, 211180868835057744408834048000, 26072812428113877344085395644416000, 4829206317935252350431489703482654720000

Offset

1,2

Links

Table of n, a(n) for n=1..11.

Formula

a(n) = ((2n)!^3 / n!^2 / 2^(2n) + (2n)!) / (4n).

a(n) = A137737(2n)

Crossrefs

Cf. A114938, A137729, A137730, A137737

Sequence in context: A294975 A294979 A250435 * A007741 A176378 A089037

Adjacent sequences: A137746 A137747 A137748 * A137750 A137751 A137752

Keyword

nonn

Author

Max Alekseyev, Feb 09, 2008

Status

approved