In the structural model Y X0e with X 0Z u and `k, ` k, we claim that a necessary condition for

In the structural model Y Æ X0¯Åe with X Æ ¡0Z Åu and ¡ `£k, ` ¸ k, we claim that a necessary condition for ¯ to be identified (can be recovered from the reduced form) is rank(¡) Æ k.

Explain why this is true. That is, show that if rank(¡) Ç k then ¯ is not identified.