Let λ be an eigenvalue of invertible matrix A. Prove that exists and is an eigenvalue of A-¹. What does your proof also say about the corresponding eigenvector?

Advanced Engineering Mathematics
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ISBN:9780470458365
Author:Erwin Kreyszig
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Chapter2: Second-order Linear Odes
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**Question:**

Let \( \lambda \) be an eigenvalue of invertible matrix \( A \). Prove that \( \frac{1}{\lambda} \) exists and is an eigenvalue of \( A^{-1} \). What does your proof also say about the corresponding eigenvector? 

**Explanation:**

This question involves concepts from linear algebra, specifically eigenvalues and eigenvectors.

1. **Definition Recall:**
   - An eigenvalue \( \lambda \) of a matrix \( A \) means there exists a non-zero vector \( \mathbf{v} \) such that:
     \[
     A \mathbf{v} = \lambda \mathbf{v}
     \]
   - Matrix \( A \) is invertible, denoting that its determinant is non-zero.

2. **Proof Concept:**
   - If \( A \mathbf{v} = \lambda \mathbf{v} \), then apply inverse \( A^{-1} \) to both sides:
     \[
     A^{-1} A \mathbf{v} = A^{-1} (\lambda \mathbf{v})
     \]
   - This simplifies to:
     \[
     \mathbf{v} = \lambda A^{-1} \mathbf{v}
     \]
   - Rearranging gives:
     \[
     A^{-1} \mathbf{v} = \frac{1}{\lambda} \mathbf{v}
     \]
   - Hence, \( \frac{1}{\lambda} \) is an eigenvalue of \( A^{-1} \).

3. **Implication for Eigenvector:**
   - The eigenvector \( \mathbf{v} \) remains the same for both \( A \) and \( A^{-1} \).

This explanation can accompany an educational website’s text to help students understand the properties of eigenvalues and eigenvectors for invertible matrices.
Transcribed Image Text:**Question:** Let \( \lambda \) be an eigenvalue of invertible matrix \( A \). Prove that \( \frac{1}{\lambda} \) exists and is an eigenvalue of \( A^{-1} \). What does your proof also say about the corresponding eigenvector? **Explanation:** This question involves concepts from linear algebra, specifically eigenvalues and eigenvectors. 1. **Definition Recall:** - An eigenvalue \( \lambda \) of a matrix \( A \) means there exists a non-zero vector \( \mathbf{v} \) such that: \[ A \mathbf{v} = \lambda \mathbf{v} \] - Matrix \( A \) is invertible, denoting that its determinant is non-zero. 2. **Proof Concept:** - If \( A \mathbf{v} = \lambda \mathbf{v} \), then apply inverse \( A^{-1} \) to both sides: \[ A^{-1} A \mathbf{v} = A^{-1} (\lambda \mathbf{v}) \] - This simplifies to: \[ \mathbf{v} = \lambda A^{-1} \mathbf{v} \] - Rearranging gives: \[ A^{-1} \mathbf{v} = \frac{1}{\lambda} \mathbf{v} \] - Hence, \( \frac{1}{\lambda} \) is an eigenvalue of \( A^{-1} \). 3. **Implication for Eigenvector:** - The eigenvector \( \mathbf{v} \) remains the same for both \( A \) and \( A^{-1} \). This explanation can accompany an educational website’s text to help students understand the properties of eigenvalues and eigenvectors for invertible matrices.
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