Chapter 7: Problem 14
Show that \(\|A\|_{F}=\left\|A^{T}\right\|_{F}\)
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Chapter 7: Problem 14
Show that \(\|A\|_{F}=\left\|A^{T}\right\|_{F}\)
These are the key concepts you need to understand to accurately answer the question.
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Let \(I\) denote the \(n \times n\) identity matrix. Determine the values of \(\|I\|_{1},\|I\|_{\infty},\) and \(\|I\|_{F}\)
Let \(A=L U,\) where \(L\) is lower triangular with 1 's on the diagonal and \(U\) is upper triangular. (a) How many scalar additions and multiplications are necessary to solve \(L \mathbf{y}=\mathbf{e}_{j}\) by forward substitution? (b) How many additions/subtractions and multiplications/divisions are necessary to solve \(A \mathbf{x}=\mathbf{e}_{j} ?\) The solution \(\mathbf{x}_{j}\) of \(A \mathbf{x}=\mathbf{e}_{j}\) will be the \(j\) th column of \(A^{-1}\) (c) Given the factorization \(A=L U\), how many additional multiplications/divisions and additions/subtractions are needed to compute \(A^{-1} ?\)
Let \(A\) and \(B\) be nonsingular \(n \times n\) matrices. Show that \\[ \operatorname{cond}(A B) \leq \operatorname{cond}(A) \operatorname{cond}(B) \\]
Let u be a unit vector in \(\mathbb{R}^{n}\) and let \\[ Q=I-2 \mathbf{u u}^{T} \\] (a) Show that \(\mathbf{u}\) is an eigenvector of \(Q .\) What is the corresponding eigenvalue? (b) Let \(\mathbf{z}\) be a nonzero vector in \(\mathbb{R}^{n}\) that is orthogonal to u. Show that \(\mathbf{z}\) is an eigenvector of \(Q\) belonging to the eigenvalue \(\lambda=1\) (c) Show that the eigenvalue \(\lambda=1\) must have multiplicity \(n-1 .\) What is the value of \(\operatorname{det}(Q) ?\)
Let \(A\) be an \(m \times n\) matrix and let \(\mathbf{b} \in \mathbb{R}^{m} .\) Show that \(\mathbf{b} \in R(A)\) if and only if \\[ \mathbf{b}=A A^{+} \mathbf{b} \\]
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