63108domain: NAppears in sequencesa(n+3) = a(n+2) + a(n+1) + a(n) - 4.at n=19A000803Rook polynomials.at n=17A004306Triangle read by rows: T(n,m) is the number of inequivalent n X m matrices under action of the Klein group, with one-third each of 1s, 2s and 3s (ordered occurrences rounded up/down if m*n != 0 mod 3).at n=30A286892