Give the necessary conditions for matrix AAA can be factorized（複選，全對才給分）
A=λ1u1u1T+λ2u2u2T+⋯+λnununTA = \lambda_1 \mathbf{u}_1\mathbf{u}_1^T + \lambda_2 \mathbf{u}_2\mathbf{u}_2^T + \cdots + \lambda_n \mathbf{u}_n\mathbf{u}_n^TA=λ1u1u1T+λ2u2u2T+⋯+λnununT.
