Math Problem Statement

Determine the eigen values and eigen vectors of the matrix A=(0 0 -2 1 2 1 1 0 3)

Solution

We are given the matrix A=(002121103)A = \begin{pmatrix} 0 & 0 & -2 \\ 1 & 2 & 1 \\ 1 & 0 & 3 \end{pmatrix}.

Step 1: Find the Eigenvalues

To find the eigenvalues of the matrix AA, we need to solve the characteristic equation:

det(AλI)=0\text{det}(A - \lambda I) = 0

where λ\lambda represents the eigenvalues and II is the identity matrix. The matrix AλIA - \lambda I is:

AλI=(002121103)λ(100010001)=(0λ0212λ1103λ)A - \lambda I = \begin{pmatrix} 0 & 0 & -2 \\ 1 & 2 & 1 \\ 1 & 0 & 3 \end{pmatrix} - \lambda \begin{pmatrix} 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{pmatrix} = \begin{pmatrix} 0-\lambda & 0 & -2 \\ 1 & 2-\lambda & 1 \\ 1 & 0 & 3-\lambda \end{pmatrix}

The determinant of AλIA - \lambda I is:

det(AλI)=λ0212λ1103λ\text{det}(A - \lambda I) = \begin{vmatrix} -\lambda & 0 & -2 \\ 1 & 2-\lambda & 1 \\ 1 & 0 & 3-\lambda \end{vmatrix}

Now, expand this determinant:

=λ2λ103λ01113λ+(2)12λ10= -\lambda \begin{vmatrix} 2-\lambda & 1 \\ 0 & 3-\lambda \end{vmatrix} - 0 \cdot \begin{vmatrix} 1 & 1 \\ 1 & 3-\lambda \end{vmatrix} + (-2) \begin{vmatrix} 1 & 2-\lambda \\ 1 & 0 \end{vmatrix}

Let's simplify each part:

  1. 2λ103λ=(2λ)(3λ)=65λ+λ2\begin{vmatrix} 2-\lambda & 1 \\ 0 & 3-\lambda \end{vmatrix} = (2-\lambda)(3-\lambda) = 6 - 5\lambda + \lambda^2
  2. 12λ10=(2λ)\begin{vmatrix} 1 & 2-\lambda \\ 1 & 0 \end{vmatrix} = - (2-\lambda)

Thus, the determinant becomes:

λ(65λ+λ2)+(2)((2λ))=λ(λ25λ+6)+2(2λ)-\lambda \left( 6 - 5\lambda + \lambda^2 \right) + (-2)(-(2-\lambda)) = -\lambda(\lambda^2 - 5\lambda + 6) + 2(2-\lambda)

Simplifying:

λ3+5λ26λ+42λ=λ3+5λ28λ+4-\lambda^3 + 5\lambda^2 - 6\lambda + 4 - 2\lambda = -\lambda^3 + 5\lambda^2 - 8\lambda + 4

We set this equal to 0 to get the characteristic equation:

λ3+5λ28λ+4=0-\lambda^3 + 5\lambda^2 - 8\lambda + 4 = 0

Step 2: Solve the Characteristic Equation

We solve this cubic equation for λ\lambda. Using trial and error or synthetic division, we find that λ=1\lambda = 1 is a root. Dividing λ3+5λ28λ+4-\lambda^3 + 5\lambda^2 - 8\lambda + 4 by λ1\lambda - 1, we get:

λ3+5λ28λ+4=(λ1)(λ2+6λ4)-\lambda^3 + 5\lambda^2 - 8\lambda + 4 = (\lambda - 1)(-\lambda^2 + 6\lambda - 4)

Now, solve λ2+6λ4=0-\lambda^2 + 6\lambda - 4 = 0 using the quadratic formula:

λ=6±624(1)(4)2(1)=6±36162=6±202=6±252\lambda = \frac{-6 \pm \sqrt{6^2 - 4(-1)(-4)}}{2(-1)} = \frac{-6 \pm \sqrt{36 - 16}}{-2} = \frac{-6 \pm \sqrt{20}}{-2} = \frac{-6 \pm 2\sqrt{5}}{-2}

Thus, the eigenvalues are:

λ1=1,λ2=35,λ3=3+5\lambda_1 = 1, \quad \lambda_2 = 3 - \sqrt{5}, \quad \lambda_3 = 3 + \sqrt{5}

Step 3: Find the Eigenvectors

For each eigenvalue λ\lambda, solve (AλI)v=0(A - \lambda I)v = 0 to find the corresponding eigenvector vv.

Eigenvalue λ1=1\lambda_1 = 1:

Solve (AI)v=0(A - I)v = 0:

(102111102)(x1x2x3)=0\begin{pmatrix} -1 & 0 & -2 \\ 1 & 1 & 1 \\ 1 & 0 & 2 \end{pmatrix} \begin{pmatrix} x_1 \\ x_2 \\ x_3 \end{pmatrix} = 0

This gives a system of equations. Solving this system yields the eigenvector corresponding to λ1=1\lambda_1 = 1.

Similarly, you can solve for the eigenvectors corresponding to λ2=35\lambda_2 = 3 - \sqrt{5} and λ3=3+5\lambda_3 = 3 + \sqrt{5}.

Would you like further steps on calculating the eigenvectors? Let me know if you want more details!

Related Questions:

  1. How do we determine the nature of eigenvalues (real or complex)?
  2. What are the properties of matrices with real eigenvalues?
  3. How do we use eigenvalues to diagonalize a matrix?
  4. What is the geometric interpretation of eigenvectors and eigenvalues?
  5. How can we solve the characteristic equation for more complex matrices?

Tip:

For 3x3 matrices, trial and error or using known factoring techniques can help solve cubic characteristic equations more efficiently.

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Math Problem Analysis

Mathematical Concepts

Linear Algebra
Eigenvalues
Eigenvectors
Characteristic Equation
Determinants
Cubic Equations

Formulas

det(A - λI) = 0
Quadratic formula: λ = (-b ± √(b² - 4ac)) / 2a

Theorems

Eigenvalue Theorem
Cayley-Hamilton Theorem

Suitable Grade Level

Undergraduate level