by Qiang Gao, updated at Apr 23, 2017
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In Proposition 1.1 and 1.2, where did we use Assumption 1.3 that $$ \mathrm{rank} ( \mathbf{X} ) = K $$?
We need the no-multicollinearity condition to make sure $$ \mathbf{X}' \mathbf{X} $$ is invertible.
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