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A137737 Number of circular permutations of the multiset {1,1,2,2,...,n,n} (up to rotations) with even distances between equal elements. 7
0, 1, 0, 30, 0, 13560, 0, 27785520, 0, 162030637440, 0, 2156625389318400, 0, 56857271240920550400, 0, 2686506065987036477184000, 0, 211180868835057744408834048000, 0, 26072812428113877344085395644416000, 0 (list; graph; refs; listen; history; text; internal format)
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

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Last modified June 20 10:00 EDT 2021. Contains 345162 sequences. (Running on oeis4.)