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Linear Modelinghard
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For the matrix form of regression Y=Xβ+ϵY = X\beta + \epsilonY=Xβ+ϵ, what is the necessary and sufficient condition for the OLS estimator β^=(XTX)−1XTY\hat{\beta} = (X^T X)^{-1} X^T Yβ^​=(XTX)−1XTY to be unique?