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A137729
Number of circular permutations of the multiset {1,1,2,2,...,n,n} (up to rotations).
11
1, 1, 2, 16, 318, 11352, 623760, 48648960, 5108105520, 694702028160, 118794043549440, 24946749109094400, 6311527524161798400, 1893458257242791500800, 664603848292138865510400, 269829162406607158901145600
OFFSET
0,3
LINKS
FORMULA
a(n) = ((2n)!/2^n + n!) / (2n).
CROSSREFS
Row n=2 of A208183.
Sequence in context: A189189 A188867 A282787 * A278589 A171212 A282392
KEYWORD
nonn
AUTHOR
Max Alekseyev, Feb 09, 2008
EXTENSIONS
a(0)=1 prepended by Alois P. Heinz, Jul 28 2015
STATUS
approved