 positive scalar λ, $Z^{*}$ is the optimal solution of* $$\min\frac{0}{0}\left| \middle| Z \right. - \left. W \middle| \right|_{F}^{0} + \left. \lambda \middle| \middle| Z \middle| \right|_{0,0},$$ *and the i-th column of $Z^{*}$ 