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).

1, 2, 6, 72, 1240, 33360

Offset

1,2

Links

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

Crossrefs

Cf. A002105, A117514, A117513.

Sequence in context: A156493 A339299 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