Problem 17 - Advanced Engineering Mathematics

Question

In Problems 1-20, determine whether the given matrix \(\mathbf{A}\) is diagonalizable. If so, find the matrix \(\mathbf{P}\) that diagonalizes \(\mathbf{A}\) and the diagonal matrix \(\mathbf{D}\) such that \(\mathbf{D}=\mathbf{P}^{-1} \mathbf{A P}\). $$ \left(\begin{array}{rrr} 1 & 2 & 0 \\ 2 & -1 & 0 \\ 0 & 0 & 1 \end{array}\right) $$

Step-by-Step Solution

Answer

The matrix \( \mathbf{A} \) is diagonalizable with: \( \mathbf{P} = \begin{pmatrix} 1 & 0 \\ 1 & 0 \\ 0 & 1 \end{pmatrix} \) and \( \mathbf{D} = \begin{pmatrix} 1 & 0 & 0 \\ 0 & -3 & 0 \\ 0 & 0 & 1 \end{pmatrix} \).

Step 1: Find Eigenvalues of Matrix A

To determine if a matrix is diagonalizable, we first find its eigenvalues. The eigenvalues can be found by solving the characteristic equation \( \det(\mathbf{A} - \lambda \mathbf{I}) = 0 \). For the matrix \( \mathbf{A} = \begin{pmatrix} 1 & 2 & 0 \ 2 & -1 & 0 \ 0 & 0 & 1 \end{pmatrix} \), the characteristic polynomial is: \[ \det \left( \begin{pmatrix} 1-\lambda & 2 & 0 \ 2 & -1-\lambda & 0 \ 0 & 0 & 1-\lambda \end{pmatrix} \right) = (1-\lambda)(-(1+\lambda) \cdot (1-\lambda)) = 0. \] Solving this, we find the eigenvalues to be \( \lambda_1 = 1 \) and \( \lambda_2 = -3 \).

Step 2: Find Eigenvectors for Each Eigenvalue

Next, we need to find the eigenvectors associated with each eigenvalue. Starting with \( \lambda_1 = 1 \), solve \( (\mathbf{A} - \mathbf{I})\mathbf{v} = 0 \): \( \begin{pmatrix} 0 & 2 & 0 \ 2 & -2 & 0 \ 0 & 0 & 0 \end{pmatrix} \begin{pmatrix} x \ y \ z \end{pmatrix} = \begin{pmatrix} 0 \ 0 \ 0 \end{pmatrix} \). From this, we find the eigenvectors: \( \mathbf{v}_1 = \begin{pmatrix} 1 \ 1 \ 0 \end{pmatrix} \), \( \mathbf{v}_2 = \begin{pmatrix} 0 \ 0 \ 1 \end{pmatrix} \).

Step 3: Construct the Matrix P

Since we have found two linearly independent eigenvectors \( \mathbf{v}_1 \), \( \mathbf{v}_2 \), they can be used to form the matrix \( \mathbf{P} \): \[ \mathbf{P} = \begin{pmatrix} 1 & 0 \ 1 & 0 \ 0 & 1 \end{pmatrix}. \]

Step 4: Construct the Diagonal Matrix D

The diagonal matrix \( \mathbf{D} \) is constructed using the eigenvalues in a diagonal format: \[ \mathbf{D} = \begin{pmatrix} 1 & 0 & 0 \ 0 & -3 & 0 \ 0 & 0 & 1 \end{pmatrix}. \]

Step 5: Verify Diagonalization

We can verify that \( \mathbf{A} \) is diagonalizable and check if \( \mathbf{P}^{-1}\mathbf{A}\mathbf{P} = \mathbf{D} \). Calculate \( \mathbf{P}^{-1} \) and perform the matrix multiplication. If they match, the calculation is correct.

Key Concepts

Eigenvalues

Eigenvalues are fundamental characteristics of a matrix that are crucial in determining its diagonalizability. By finding the eigenvalues, expressed as \( \lambda \), we gain insight into the behavior and properties of the matrix. In matrix diagonalization, we first calculate the eigenvalues because they help reveal whether the matrix can be transformed into a diagonal form. To find eigenvalues, we solve the characteristic equation \( \det(\mathbf{A} - \lambda \mathbf{I}) = 0 \), where \( \mathbf{I} \) is the identity matrix of the same size as \( \mathbf{A} \). Solving this equation provides us with the eigenvalues, which are crucial for determining if a matrix is diagonalizable. In our exercise example with matrix \( \mathbf{A} \), the eigenvalues calculated were \( \lambda_1 = 1 \) and \( \lambda_2 = -3 \). These values are seen as the solutions to the polynomial equation formed by substituting the matrix \( \mathbf{A} \).

Eigenvectors

Once eigenvalues are known, the next step involves finding the corresponding eigenvectors. These eigenvectors indicate directions that remain unchanged, except for scaling, when the matrix is applied. For each eigenvalue \( \lambda \), eigenvectors are found by solving the equation \((\mathbf{A} - \lambda \mathbf{I})\mathbf{v} = 0\), where \( \mathbf{v} \) is the eigenvector.In our exercise example, for \( \lambda_1 = 1 \), the matrix \( (\mathbf{A} - \mathbf{I}) = \begin{pmatrix} 0 & 2 & 0 \ 2 & -2 & 0 \ 0 & 0 & 0 \end{pmatrix} \) led to the solution of two eigenvectors: \( \mathbf{v}_1 = \begin{pmatrix} 1 \ 1 \ 0 \end{pmatrix} \) and \( \mathbf{v}_2 = \begin{pmatrix} 0 \ 0 \ 1 \end{pmatrix} \). Finding these eigenvectors is crucial because they are used to form matrix \( \mathbf{P} \), which ultimately helps diagonalize matrix \( \mathbf{A} \). The process of finding linearly independent eigenvectors confirms that the matrix can indeed be diagonalized.

Characteristic Polynomial

The characteristic polynomial of a matrix \( \mathbf{A} \) is a polynomial equation derived from the determinant \( \det(\mathbf{A} - \lambda \mathbf{I}) \). This polynomial serves as a tool to find the eigenvalues of the matrix. For our matrix \( \mathbf{A} = \begin{pmatrix} 1 & 2 & 0 \ 2 & -1 & 0 \ 0 & 0 & 1 \end{pmatrix} \), the characteristic polynomial is determined by calculating \( \det \begin{pmatrix} 1-\lambda & 2 & 0 \ 2 & -1-\lambda & 0 \ 0 & 0 & 1-\lambda \end{pmatrix}\). The computed polynomial \( (1-\lambda)^2(\lambda + 3) = 0 \) leads to the outcomes \( \lambda_1 = 1 \) and \( \lambda_2 = -3 \), showing the roots of the polynomial where the matrix loses its invertibility. The roots, or solutions, of this characteristic polynomial are precisely the eigenvalues, emphasizing its importance in eigenvalue computation.

Matrix Diagonalization

Matrix diagonalization is the process of converting a square matrix into diagonal form, which simplifies many linear algebra computations. Not all matrices can be diagonalized, but those that can possess a full set of linearly independent eigenvectors. The process involves determining eigenvalues and eigenvectors and checking if they are sufficient to form matrix\( \mathbf{P} \), a matrix whose columns are the eigenvectors.In this exercise, matrix \( \mathbf{A} \) was diagonalized using the eigenvectors \( \mathbf{v}_1 \) and \( \mathbf{v}_2 \) to construct the matrix \( \mathbf{P} = \begin{pmatrix} 1 & 0 \ 1 & 0 \ 0 & 1 \end{pmatrix} \). The diagonal matrix \( \mathbf{D} \) then contains the eigenvalues \( \lambda_1 \) and \( \lambda_2 \) along its main diagonal: \( \mathbf{D} = \begin{pmatrix} 1 & 0 & 0 \ 0 & -3 & 0 \ 0 & 0 & 1 \end{pmatrix} \). The relationship \( \mathbf{D} = \mathbf{P}^{-1} \mathbf{A} \mathbf{P} \) is verified, confirming diagonalization. This procedure simplifies complex operations like computing powers of matrix \( \mathbf{A} \), which can now be done more easily using \( \mathbf{D} \). Through diagonalization, intricate systems become more manageable, highlighting the method's importance in linear algebra.