Your textbook derives the OLS estimator as $\hat { \boldsymbol { \beta } } = \left( \boldsymbol { X } ^ { \prime } \boldsymbol { X } \right) ^ { - 1 } \boldsymbol { X } ^ { \prime } \boldsymbol { Y }$ Show that the estimator does not exist if there are fewer observations than the number of
explanatory variables, including the constant.What is the rank of X′X in this case?

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The Answer of Your textbook derives the OLS estimator as $\hat { \boldsymbol { \beta } } = \left( \boldsymbol { X } ^ { \prime } \boldsymbol { X } \right) ^ { - 1 } \boldsymbol { X } ^ { \prime } \boldsymbol { Y }$ Show that the estimator does not exist if there are fewer observations than the number of
explanatory variables, including the constant.What is the rank of X′X in this case?

Your Textbook Derives the OLS Estimator As Show That the Estimator Does Not Exist If There Are