63804560820domain: NAppears in sequencesa(n) = (1/(9n-3))*M(3n; n,n,n), where M() is a multinomial coefficient.at n=9A024489Number of permutations of 0..floor((n*3-2)/2) on odd squares of an n X 3 array such that each row and column of odd squares is increasing.at n=18A215294