Chapter 5: Problem 6
Find the point on the line \(y=2 x+1\) that is closest to the point (5,2)
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Chapter 5: Problem 6
Find the point on the line \(y=2 x+1\) that is closest to the point (5,2)
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Let \(S\) be the subspace of \(\mathbb{R}^{4}\) spanned by \(\mathbf{x}_{1}=\) \((1,0,-2,1)^{T}\) and \(\mathbf{x}_{2}=(0,1,3,-2)^{T}\). Find a ba-
Let \(P=A\left(A^{T} A\right)^{-1} A^{T},\) where \(A\) is an \(m \times n\) matrix of rank \(n\) (a) Show that \(P^{2}=P\) (b) Prove that \(P^{k}=P\) for \(k=1,2, \ldots\) (c) Show that \(P\) is symmetric. [Hint: If \(B\) is nonsingular, then \(\left.\left(B^{-1}\right)^{T}=\left(B^{T}\right)^{-1} .\right]\)
In \(C[0,1],\) with inner product defined by \((3),\) compute (a) \(\left\langle e^{x}, e^{-x}\right\rangle\) (b) \(\langle x, \sin \pi x\rangle\) (c) \(\left\langle x^{2}, x^{3}\right\rangle\)
Let \(\mathbf{x}=(5,2,4)^{T}\) and \(\mathbf{y}=(3,3,2)^{T} .\) Compute \(\|\mathbf{x}-\mathbf{y}\|_{1},\|\mathbf{x}-\mathbf{y}\|_{2},\) and \(\|\mathbf{x}-\mathbf{y}\|_{\infty} .\) Under which norm are the two vectors closest together? Under which norm are they farthest apart?
Show that if $$\left(\begin{array}{cc} A & I \\ O & A^{T} \end{array}\right)\left(\begin{array}{l} \hat{\mathbf{x}} \\ \mathbf{r} \end{array}\right)=\left(\begin{array}{l} \mathbf{b} \\ \mathbf{0} \end{array}\right)$$ then \(\hat{\mathbf{x}}\) is a least squares solution of the system \(A \mathbf{x}=\mathbf{b}\) and \(\mathbf{r}\) is the residual vector.
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