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A117515
Number of ways of arranging 2n tokens in a row, with 2 copies of each token from 1 through n, such that between every pair of tokens labeled i (i=1..n) there is exactly one taken labeled (i+1 mod n).
3
1, 2, 6, 72, 1240, 33360
OFFSET
1,2
CROSSREFS
KEYWORD
nonn
AUTHOR
Nan Zang (nzang(AT)cs.ucsd.edu), Apr 28 2006
STATUS
approved