A117515 - OEIS

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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).

%I #1 May 19 2006 03:00:00

%S 1,2,6,72,1240,33360

%N 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).

%Y Cf. A002105, A117514, A117513.

%K nonn

%O 1,2

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