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Let \(A\) be an \(n \times n\) matrix whose characteristic polynomial splits, \(\gamma\) be a cycle of generalized eigenvectors corresponding to an eigenvalue \(\lambda\), and \(W\) be the subspace spanned by \(\gamma\). Define \(\gamma^{\prime}\) to be the ordered set obtained from \(\gamma\) by reversing the order of the vectors in \(\gamma\). (a) Prove that \(\left[\mathrm{T}_{\mathrm{W}}\right]_{\gamma^{\prime}}=\left(\left[\mathrm{T}_{\mathrm{W}}\right]_{\gamma}\right)^{t}\). (b) Let \(J\) be the Jordan canonical form of \(A\). Use (a) to prove that \(J\) and \(J^{t}\) are similar. (c) Use (b) to prove that \(A\) and \(A^{t}\) are similar.

Short Answer

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In summary, we proved that for a given matrix A and its cycle of generalized eigenvectors 饾浘, the matrix representations of the linear transformation T_W restricted to the subspace W spanned by 饾浘 and the reversed order of 饾浘 are transposes of each other. Using this result, we showed that the Jordan canonical form J of A and its transpose J^t are similar matrices. Consequently, we proved that A and its transpose A^t are also similar matrices.

Step by step solution

01

Properties of matrix representation of linear transformation T_W

First, let's recall the properties of the matrix representation of a linear transformation T_W induced by matrix A in subspace W spanned by a cycle of generalized eigenvectors 饾浘. Given a cycle 饾浘 = {v_1, v_2, ..., v_k}, we know that T_W(v_i) = v_{i+1} (for 1 鈮 i 鈮 k-1) and T_W(v_k) = 饾渾v_k. Hence, the matrix representation of 馃泦-linear transformation T_W with respect to 饾浘 can be written as follows: \[ \left[\mathrm{T}_{\mathrm{W}}\right]_{\gamma} = \begin{pmatrix} \lambda & 1 & & & \\ & \lambda & 1 & & \\ & & \lambda & \ddots & \\ & & & \ddots & 1 \\ & & & & \lambda \end{pmatrix} \] Now, let's find the matrix representation of the same transformation T_W with respect to the ordered set 饾浘' which is obtained by reversing the order of vectors in 饾浘.
02

Finding the matrix representation of T_W with respect to 饾浘'

For each vector v_i in the reversed ordered set 饾浘', we can write the generalized eigenvector equation: \(T_W(v_i) = T_W(v_{k-i+1}) = v_{k-i} = v_{i-1}\) (for 2 鈮 i 鈮 k) and \(T_W(v_k) = T_W(v_1) = \lambda v_1\) Using these relationships, we can write the matrix representation of the linear transformation T_W with respect to 饾浘' as follows: \[ \left[\mathrm{T}_{\mathrm{W}}\right]_{\gamma^{\prime}} = \begin{pmatrix} \lambda & & & & \\ 1 & \lambda & & & \\ & 1 & \lambda & & \\ & & \ddots & \ddots & \\ & & & 1 & \lambda \end{pmatrix} \] Now observe that the matrix \(\left[\mathrm{T}_{\mathrm{W}}\right]_{\gamma^{\prime}}\) is indeed the transpose of the matrix \(\left[\mathrm{T}_{\mathrm{W}}\right]_{\gamma}\). Therefore, we have proven part (a) of the problem: \(\left[\mathrm{T}_{\mathrm{W}}\right]_{\gamma^{\prime}}=\left(\left[\mathrm{T}_{\mathrm{W}}\right]_{\gamma}\right)^{t}\) #Part b: Proving that J and J^t are similar matrices#
03

Using the result from part (a) to prove the similarity between J and J^t

Let J be the Jordan canonical form of the matrix A. By definition, every Jordan block in J has the same structure as the matrix representation of T_W with respect to 饾浘, found in part (a). Thus, the transpose of J, denoted by J^t, will have the same structure as the matrix representation of T_W with respect to 饾浘'. Now, since the matrices \(\left[\mathrm{T}_{\mathrm{W}}\right]_{\gamma}\) and \(\left[\mathrm{T}_{\mathrm{W}}\right]_{\gamma^{\prime}}\) are the transposes of each other, their respective similarity matrices will also be the transposes of each other. Therefore, applying the same similarity transformation on J and J^t, we obtain that J and J^t are similar matrices. #Part c: Proving that A and A^t are similar matrices#
04

Proving the similarity between A and A^t using the result from part (b)

Since the Jordan canonical form J of the matrix A and its transpose J^t are similar matrices, there exists an invertible matrix P, such that: \(J^t = P^{-1}JP\) Now, since A is similar to J, there exists an invertible matrix Q, such that: \(J = Q^{-1}AQ\) Taking the transpose of the above equation, we get: \(J^t = (AQ)^t = Q^t A^t (Q^{-1})^t\) Comparing this with the equation involving P, we can conclude: \(A^t = (Q^{-1})^t J^t Q^t = (Q^{-1})^t P^{-1}JP Q^t\) which shows that A^t is similar to A when using matrix \((Q^{-1})^t P^{-1}\) as the similarity transformation. Thus, we have proven that A and A^t are similar matrices as well.

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Most popular questions from this chapter

The following definition is used in Exercises 20 and \(21 .\) Definition. For any \(A \in \mathrm{M}_{n \times n}(C)\), define the norm of \(A\) by $$ \|A\|_{m}=\max \left\\{\left|A_{i j}\right|: 1 \leq i, j \leq n\right\\} . $$ Let \(A, B \in \mathrm{M}_{n \times n}(C)\). Prove the following results. (a) \(\|A\|_{m} \geq 0\). (b) \(\|A\|_{m}=0\) if and only if \(A=O\). (c) \(\|c A\|_{m}=|c| \cdot\|A\|_{m}\) for any scalar \(c\). (d) \(\|A+B\|_{m} \leq\|A\|_{m}+\|B\|_{m}\). (e) \(\|A B\|_{m} \leq n\|A\|_{m}\|B\|_{m}\).

Find the minimal polynomial of each of the following matrices. (a) \(\left(\begin{array}{ll}2 & 1 \\ 1 & 2\end{array}\right)\) (b) \(\left(\begin{array}{ll}1 & 1 \\ 0 & 1\end{array}\right)\) (c) $\left(\begin{array}{rrr}4 & -14 & 5 \\ 1 & -4 & 2 \\ 1 & -6 & 4\end{array}\right)$ (d) $\left(\begin{array}{rrr}3 & 0 & 1 \\ 2 & 2 & 2 \\ -1 & 0 & 1\end{array}\right)$

Consider the subspace \(W=\mathbf{P}_{2}(t)\) of \(\mathbf{P}(t)\) with inner product \(\langle f, g\rangle=\int_{0}^{1} f(t) g(t) d t .\) Find the projection of \(f(t)=t^{3}\) onto \(W\). (Hint: Use the orthogonal polynomials \(1,2 t-1,6 t^{2}-6 t+1\) obtained in Problem \(7.22 .\) )

Prove Theorem 7.15: Let \(A\) be a real positive definite matrix. Then the function \(\langle u, v\rangle=u^{T} A v\) is an inner product on \(\mathbf{R}^{n}\) For any vectors \(u_{1}, u_{2},\) and \(v\) \\[\left\langle u_{1}+u_{2}, \quad v\right\rangle=\left(u_{1}+u_{2}\right)^{T} A v=\left(u_{1}^{T}+u_{2}^{T}\right) A v=u_{1}^{T} A v+u_{2}^{T} A v=\left\langle u_{1}, v\right\rangle+\left\langle u_{2}, v\right\rangle\\] and, for any scalar \(k\) and vectors \(u, v\) \\[\langle k u, v\rangle=(k u)^{T} A v=k u^{T} A v=k\langle u, v\rangle\\] Thus \(\left[\mathrm{I}_{1}\right]\) is satisfied. Because \(u^{T} A v\) is a scalar, \(\left(u^{T} A v\right)^{T}=u^{T} A v .\) Also, \(A^{T}=A\) because \(A\) is symmetric. Therefore, \\[\langle u, v\rangle=u^{T} A v=\left(u^{T} A v\right)^{T}=v^{T} A^{T} u^{T T}=v^{T} A u=\langle v, u\rangle\\] Thus, \(\left[\mathrm{I}_{2}\right]\) is satisfied. Last, because \(A\) is positive definite, \(X^{T} A X>0\) for any nonzero \(X \in \mathbf{R}^{n}\). Thus, for any nonzero vector \(v,\langle v, v\rangle=v^{T} A v>0 .\) Also, \(\langle 0,0\rangle=0^{T} A 0=0 .\) Thus, \(\left[\mathrm{I}_{3}\right]\) is satisfied. Accordingly, the function \(\langle u, v\rangle=A v\) is an inner product.

Prove (a) \(\|\cdot\|_{1}\) is a norm on \(\mathbf{R}^{n}\). (b) \(\|\cdot\|_{\infty}\) is a norm on \(\mathbf{R}^{n}\).

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