35199360domain: NAppears in sequencesE.g.f. (1-2x)/(1-2x-x^3).at n=9A052639Number of pairs (x,y) of elements x of the symmetric group S_{n-1} and y of the symmetric group S_{n} that commute. Here the symmetric group S_{n-m} is to be thought of as the subgroup of the symmetric group S_n which stabilizes n-m+1,n-m+2,...n.at n=9A225108