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Problem 222

[A] Find the minimum polynomial \(\mathrm{m}(\lambda)\) of the matrix $\mathrm{A}=\begin{array}{cccc}2 & 1 & 0 & 0 \\ & 0 & 2 & 0 & 0 \mid \\ & 0 & 0 & 2 & 0 \\ & 0 & 0 & 0 & 5\end{array}$ [B] Let \(\mathrm{A}\) be a 3 by 3 matrix over the real field cannot be a zero of the polynomial, \(\varphi(\lambda)=\lambda^{2}+1\)

Short Answer

Expert verified
The minimum polynomial of the given matrix is \(m(\lambda) = (\lambda - 2)^{2}(\lambda - 5)\). For a 3x3 matrix A, it is impossible for φ(λ) to be the characteristic polynomial. An example of a 3x3 real matrix that is not a zero of φ(λ) is the identity matrix with the characteristic polynomial (λ - 1)^3.
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Step 1: Find Eigenvalues and their Multiplicities

Since the given matrix is diagonal, the eigenvalues are the diagonal entries. So, we have three eigenvalues: λ1 = 2, λ2 = 2, and λ3 = 5. Since λ1 and λ2 are equal, let's combine them as just λ1 with algebraic multiplicity 2.

Step 2: Construct the Minimum Polynomial

Now, we will construct the minimum polynomial m(λ) using the eigenvalues and their algebraic multiplicities. m(λ) must be the product of unique linear factors corresponding to distinct eigenvalues, each raised to their algebraic multiplicity. m(λ) = (λ - λ1)^2(λ - λ3) m(λ) = (λ - 2)^2(λ - 5) So the minimum polynomial is: \[m(\lambda) = (\lambda - 2)^{2}(\lambda - 5)\] [B]

Step 1: Find the Characteristic Polynomial

A 3x3 matrix A will have a characteristic polynomial of degree 3. We will call it χ(λ) = c_0 + c_1λ + c_2λ^2 + λ^3.

Step 2: Identify Matrices Where φ(λ) is not the Characteristic Polynomial

We know that φ(λ) = λ^2 + 1. For a 3x3 matrix A, it is impossible for φ(λ) to be the characteristic polynomial. This is because the characteristic polynomial of a 3x3 matrix must have degree 3 while φ(λ) has degree 2.

Step 3: Give an Example

Consider the 3x3 identity matrix: \[A = \begin{bmatrix} 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{bmatrix}\] The characteristic polynomial of A is: χ(λ) = (λ - 1)^3 This matrix A is an example of a 3x3 real matrix that is not a zero of φ(λ).

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Most popular questions from this chapter

Chapter 9

Show that the following theorem is true: If two matrices are similar, then they have the same characteristic polynomial. Then show, by means of a counter-example, that the converse is false.

Chapter 9

Let $\mathrm{f}(\mathrm{x})=2 \mathrm{x}^{4}+\mathrm{x}^{3}+4 \mathrm{x}^{2}+3 \mathrm{x}+1\( and \)\mathrm{g}(\mathrm{x})=\mathrm{x}^{2}+1$. Find the polynomials \(\mathrm{q}(\mathrm{x})\) and \(\mathrm{r}(\mathrm{x})\) such that $\mathrm{f}(\mathrm{x})=\mathrm{q}(\mathrm{x}) \mathrm{g}(\mathrm{x})+\mathrm{r}(\mathrm{x})$

Chapter 9

Let \(\quad \varphi(\lambda)=-2-5 \lambda+3 \lambda^{2}\) $$ \begin{array}{rrrr}A= & \mid 1 & 2 \mid . & \text { Show that } & \varphi(A)=\mid 14 & 2 \mid \\ & \mid 3 & 1 \mid & & 13 & 14\end{array} $$

Chapter 9

Find the minimal polynomials of the following matrices: (i) \(\begin{array}{cc}\mid 3 & 1 \mid \\ \mid 0 & 3 \mid\end{array}\) (ii) $\begin{array}{ccc}\mid 3 & 1 & 0 \\ & \mid 0 & 3 & 0 \mid \\ & \mid 0 & 0 & 3 \mid\end{array}$ (iii) $\begin{array}{ccc}\mid 2 & 0 & 0 \\ & \mid 0 & 3 & 1 \mid \\ \mid 0 & 0 & 3 \mid\end{array}$ (iv) $\begin{array}{rrrr} & 2 & 0 & 0 & 0 \\ & \mid 0 & 2 & 0 & 0 \mid \\ & \mid 0 & 0 & 3 & 0 \\ & 0 & 0 & 0 & 3\end{array}$

Chapter 9

Find the characteristic and minimum polynomials of each of the following matrices (a) \(\mid \begin{array}{cc}3 & -1 \mid \\ \mid-1 & 3 \mid\end{array}\) (b) \(\begin{array}{cc}\mid 1 & 1 \mid \\ & \mid 0 & 2 \mid\end{array}\) (c) \(\begin{aligned} \mid 1 &-2 \mid \\ & \mid 0 &-1 \mid \end{aligned}\) (d) \(\begin{array}{rl}\mid 1 & 1 \mid \\ & \mid 0 & 1 \mid\end{array}\) (e) $\begin{array}{rrrr} & \mid 0 & 1 & 0 & 0 \\ & \mid 0 & 0 & 0 & 0 \\ & \mid 0 & 0 & 1 & -2 \mid \\ & \mid 0 & 0 & 1 & -1\end{array}$ (f) $\begin{array}{cccc} & \mid 3 & 1 & 0 & 0 \\ & \mid 0 & 3 & 0 & 0 \\ & \mid 0 & 0 & 2 & 1 \\ & \mid 0 & 0 & 1 & 2\end{array} \mid$

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