222965domain: NAppears in sequencesa(n) = n*(n-1)*a(n-1)/2 + a(n-2), a(0) = 1, a(1) = 2.at n=7A001052Total preorders.at n=11A006328Number of 6-bead necklaces labeled with numbers -n..n not allowing reversal, with sum zero and avoiding the patterns z z+1 z+2 and z z-1 z-2.at n=8A209118