Chess board a b d e f g h 8 × 8 For a particular set of X 7 forbidden positions 6 X 宣 6 5 5 宣X Bc[ml×[ml 4 4 3 × 3 2 2 宣口 X 1 a b d e g h No the of placements of n non-attacking rooks?Chess board B ⇥ [n] ￾ [n] For a particular set of forbidden positions the # of placements of n non-attacking rooks? N0 :