%I A117515
%S A117515 1,2,6,72,1240,33360
%N 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).
%Y A117515 Cf. A002105, A117514, A117513.
%Y A117515 Sequence in context: A152885 A052613 A156493 this_sequence A085865 A061296 
               A019993
%Y A117515 Adjacent sequences: A117512 A117513 A117514 this_sequence A117516 A117517 
               A117518
%K A117515 nonn
%O A117515 1,2
%A A117515 Nan Zang (nzang(AT)cs.ucsd.edu), Apr 28 2006