A137737 Number of circular permutations of the multiset {1,1,2,2,...,n,n} (up to rotations) with even distances between equal elements.

0, 1, 0, 30, 0, 13560, 0, 27785520, 0, 162030637440, 0, 2156625389318400, 0, 56857271240920550400, 0, 2686506065987036477184000, 0, 211180868835057744408834048000, 0, 26072812428113877344085395644416000, 0

Offset

1,4

Links

Table of n, a(n) for n=1..21.

Formula

For odd n, a(n) = 0. For even n, a(n) = (n!^3 / (n/2)!^2 / 2^n + n!) / (2n).

a(2n) = A137749(n)

Crossrefs

Cf. A114938, A137729, A137730, A137749

Sequence in context: A198805 A030128 A137348 * A262706 A062513 A040906

Adjacent sequences: A137734 A137735 A137736 * A137738 A137739 A137740

Keyword

nonn

Author

Max Alekseyev, Feb 09, 2008

Status

approved