the minimization problem is \min_{\Lambda, F_t} \frac{1}{NT} \sum_{t=1}^T (X_t - \Lambda F_t)' \Sigma_{ee}^{-1} (X_t - \Lambda F_t) s.t. \Lambda \Lambda' = I by replacing F_t = (\Lambda' \Sigma_{ee}^{-1} \Lambda)^{-1} \Lambda' \Sigma_{ee}^{-1} X_t