Let $A$ be a matrix belonging to $\mathbf{GL}_n$. Let $x$ be a nonzero real number. Express $\det(x I_n - A)$ in terms of $x$, $\det A$ and $\det\left(\frac{1}{x} I_n - A^{-1}\right)$.

Let $A$ be a matrix belonging to $\mathbf{GL}_n$. Let $x$ be a nonzero real number. Express $\det(x I_n - A)$ in terms of $x$, $\det A$ and $\det\left(\frac{1}{x} I_n - A^{-1}\right)$.

Deduce, using question 6, that if $p$ has a stable root then $J(p)$ is not invertible.

Let two matrices $A, B \in \mathcal{M}_n(\mathbf{R})$ such that there exists a matrix $P \in GL_n(\mathbf{R})$ satisfying $A = P^\top B P$. Show that $d(B) \geq d(A)$ then that $d(B) = d(A)$.

Deduce that the matrix $J(p)$ is invertible if and only if $p$ has no stable root.

We assume in this part that all roots of $p$ are stable and have multiplicity 1 and we denote by $h = Xp'$ (where $p'$ is the derivative polynomial of $p$) and $h_0$ the reciprocal polynomial of $h$. We recall that, according to question 3, there exists a real number $\lambda \in \{-1, 1\}$ such that $p = \lambda p_0$.
Show that $J(h)$ is invertible.

Let $\mathbf{u}, \mathbf{v} \in \mathbb{R}^n \backslash \{\mathbf{0}\}$. We set $M = \mathbf{u v}^T$. Show that $M$ is a square matrix of size $n \times n$, of rank 1.

Calculate with justification the rank of the following matrix $J \in \mathcal{M}_n(\mathbb{R})$: $$J = \left(\begin{array}{cccc} 1 & 1 & \cdots & 1 \\ 1 & 1 & \cdots & 1 \\ \vdots & \vdots & \ddots & \vdots \\ 1 & 1 & \cdots & 1 \end{array}\right)$$

Conversely, let $K \in \mathcal{M}_n(\mathbb{R})$ be a square matrix of rank 1. Show that there exist $\mathbf{u}, \mathbf{v} \in \mathbb{R}^n \backslash \{\mathbf{0}\}$ such that $K = \mathbf{u v}^T$.

Let $A \in \mathrm{GL}_n(\mathbb{R})$ be an invertible matrix, and let $\mathbf{u}, \mathbf{v} \in \mathbb{R}^n$. Show that $$\operatorname{det}\left(\mathbb{I}_n + \mathbf{u}\mathbf{v}^T\right) = 1 + \langle \mathbf{v}, \mathbf{u} \rangle$$

Let $A \in \mathrm{GL}_n(\mathbb{R})$ be an invertible matrix, and let $\mathbf{u}, \mathbf{v} \in \mathbb{R}^n$. Show more generally that $$\operatorname{det}\left(A + \mathbf{u}\mathbf{v}^T\right) = \operatorname{det}(A)\left(1 + \left\langle \mathbf{v}, A^{-1}\mathbf{u} \right\rangle\right).$$

Let $A \in \mathrm{GL}_n(\mathbb{R})$ be an invertible matrix, and let $\mathbf{u}, \mathbf{v} \in \mathbb{R}^n$. Let $C \in \mathcal{M}_n(\mathbb{R})$ be a matrix such that $\operatorname{det}(C) = 0$. Is it always true that $\operatorname{det}\left(C + \mathbf{u v}^T\right) = 0$? Justify your answer.

We denote by $\chi_A(x) = \operatorname{det}\left(x \mathbb{I}_n - A\right)$ the characteristic polynomial of $A$, and $\chi_B(x) = \operatorname{det}\left(x \mathbb{I}_n - B\right)$ that of $B$. Show that, for all $x \in \mathbb{R} \backslash \left\{\lambda_1, \ldots, \lambda_n\right\}$, we have $$\chi_B(x) = \chi_A(x)\left(1 - \sum_{k=1}^n \frac{\left\langle \mathbf{w}_k, \mathbf{u} \right\rangle^2}{x - \lambda_k}\right).$$

Let $\mathbf{u}, \mathbf{v} \in \mathbb{R}^n \backslash \{\mathbf{0}\}$. We set $M = \mathbf{u v}^T$. Show that $M$ is a square matrix of size $n \times n$, of rank 1.

Calculate with justification the rank of the following matrix $J \in \mathcal{M}_n(\mathbb{R})$: $$J = \left(\begin{array}{cccc} 1 & 1 & \cdots & 1 \\ 1 & 1 & \cdots & 1 \\ \vdots & \vdots & \ddots & \vdots \\ 1 & 1 & \cdots & 1 \end{array}\right).$$

Let $A \in \mathrm{GL}_n(\mathbb{R})$ be an invertible matrix, and let $\mathbf{u}, \mathbf{v} \in \mathbb{R}^n$. Show that $$\operatorname{det}\left(\mathbb{I}_n + \mathbf{u v}^T\right) = 1 + \langle \mathbf{v}, \mathbf{u} \rangle.$$

Let $A \in \mathrm{GL}_n(\mathbb{R})$ be an invertible matrix, and let $\mathbf{u}, \mathbf{v} \in \mathbb{R}^n$. Show more generally that $$\operatorname{det}\left(A + \mathbf{u v}^T\right) = \operatorname{det}(A)\left(1 + \left\langle \mathbf{v}, A^{-1} \mathbf{u} \right\rangle\right).$$

Let $A \in \mathrm{GL}_n(\mathbb{R})$ be an invertible matrix, and let $\mathbf{u}, \mathbf{v} \in \mathbb{R}^n$. Let $C \in \mathcal{M}_n(\mathbb{R})$ be a matrix such that $\operatorname{det}(C) = 0$. Is it always true that $\operatorname{det}\left(C + \mathbf{u v}^T\right) = 0$? Justify your answer.

We consider $A \in \mathcal{S}_n(\mathbb{R})$ symmetric with eigenvalues $\lambda_1 \leqslant \cdots \leqslant \lambda_n$ and corresponding orthonormal basis of eigenvectors $\left(\mathbf{w}_1, \ldots, \mathbf{w}_n\right)$. We set $B = A + \mathbf{u u}^T$ with $\|\mathbf{u}\| = 1$. We denote by $\chi_A(x) = \operatorname{det}\left(x \mathbb{I}_n - A\right)$ the characteristic polynomial of $A$, and $\chi_B(x) = \operatorname{det}\left(x \mathbb{I}_n - B\right)$ that of $B$. Show that, for all $x \in \mathbb{R} \backslash \left\{\lambda_1, \ldots, \lambda_n\right\}$, we have $$\chi_B(x) = \chi_A(x)\left(1 - \sum_{k=1}^n \frac{\left\langle \mathbf{w}_k, \mathbf{u} \right\rangle^2}{x - \lambda_k}\right).$$

Prove that the number of blocks of each type is determined by the data of the three dimensions $d_j = \dim V_j$ ($1 \leqslant j \leqslant 3$) and the three ranks $r_1 = \operatorname{rg} u_1$, $r_2 = \operatorname{rg} u_2$ and $r_{21} = \operatorname{rg}(u_2 \circ u_1)$.

In this question, we assume that $M$ is invertible. Prove that $m = n$ and that $A$ and $B$ are invertible.

137. Matrix $A = \begin{bmatrix} 5 & 2 & -1 \\ 4 & 3 & -2 \\ 1 & 6 & 7 \end{bmatrix}$ is written as the sum of a symmetric matrix and a skew-symmetric matrix. The determinant of the symmetric matrix is:
(1) $16$ (2) $18$ (3) $22$ (4) $24$

139- The values of $x$ from the relation $\begin{vmatrix} 0 & x-3 & x-2 \\ x+3 & 0 & -4 \\ x+2 & 6 & 0 \end{vmatrix} = 0$ are which of the following?
(1) $-1, -6$ (2) $-1, 6$ (3) $1, -6$ (4) $1, 6$
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132- If $A$ is a $3 \times 3$ matrix and $|A| = 4$, then the determinant of matrix $A \cdot A$ is which of the following?
(1) $64$ (2) $96$ (3) $128$ (4) $256$

33. If $A = \begin{bmatrix} \log_5^2 & \log_5^2 \\ \log_5^2 & \log_5^2 \end{bmatrix}$ and $B = \begin{bmatrix} 6|A| & 2|A| \\ 3|A| & 36|A| \end{bmatrix}$, what is the determinant of $B$?

• [(1)] $\dfrac{9}{4}$
• [(2)] $\dfrac{15}{4}$
• [(3)] $\dfrac{9}{8}$
• [(4)] $\dfrac{15}{8}$