逆矩阵存在的条件 A square matrix has full rank if and only if it is nonsingular. A matrix A has full column rank if and only if it does not have a null vector. A square matrix A is singular if and only if it has a null vector. An nx n matrix A is singular if and only if det(A)=0 这是什么意思？逆矩阵存在的条件 这是什么意思？