必要性:∑A收敛 =0 6+(4)+(42)+…+(4)+…收敛→ (A)→0A=(4);)→0 r(A)<1r(A)  1  ij + (A) ij + (A 2 ) ij ++ (A k ) ij +收 敛 ( ) ij → 0 k A 必要性：  收敛  k=0 k A = (( ) ij) → 0 k k A A