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第6章第5题 Suppose there are dd items. We first choose kk of the items to form the left-hand side of the rule. There are (dk){d \choose k} ways for doing this. After selecting the items for the left-hand

if xx is a row vector,then xTx=(x⋅x)=∥x∥22=tr(xTx)x^Tx=(x \cdot x)=\lVert x \rVert^2_2=tr(x^Tx) Linear regression: trA=trATtrA=trA^T tr(A+B)=trA+trBtr(A+B)=trA+trB traA=atrAtraA=atrA ∇AtrAB=BT\nab