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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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Table of n, a(n) for n=1..6.

CROSSREFS

Cf. A002105, A117514, A117513.

Sequence in context: A052613 A156493 A218891 * A265936 A085865 A300099

Adjacent sequences:  A117512 A117513 A117514 * A117516 A117517 A117518

KEYWORD

nonn

AUTHOR

Nan Zang (nzang(AT)cs.ucsd.edu), Apr 28 2006

STATUS

approved

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Last modified October 15 08:45 EDT 2018. Contains 316210 sequences. (Running on oeis4.)