Let a be a nonzero vector in \( \mathbb{R}^{2} \). Define \( \tau_{\mathbf{a}}: \mathbb{R}^{2} \rightarrow \mathbb{R}^{2} \) by \[ \tau_{\mathbf{a}}(\mathbf{x})=-\mathbf{x}+2 \frac{\mathbf{a} \cdot \mathbf{x}}{\mathbf{a} \cdot \mathbf{a}} \mathbf{a} . \] 2. Let a be a nonzero vector of \( \mathbb{R}^{2} \). Answer the following questions (iv) Find all eigen-values of \( \left[\tau_{\mathbf{a}}\right]_{\mathcal{B}}^{\mathcal{B}} \).
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To find the eigenvalues of the transformation \( \tau_{\mathbf{a}} \) defined by \[ \tau_{\mathbf{a}}(\mathbf{x}) = -\mathbf{x} + 2 \frac{\mathbf{a} \cdot \mathbf{x}}{\mathbf{a} \cdot \mathbf{a}} \mathbf{a}, \] we first note that \( \tau_{\mathbf{a}} \) represents a reflection with respect to the line through the origin in the direction of vector \( \mathbf{a} \). In \( \mathbb{R}^{2} \), the eigenvalues of a reflection transformation are well-known. Specifically, the eigenvalues of such transformations are: 1. \( \lambda_1 = 1 \), corresponding to the eigenvector aligned with \( \mathbf{a} \) (the line of reflection). 2. \( \lambda_2 = -1 \), corresponding to the eigenvector orthogonal to \( \mathbf{a} \) (the line perpendicular to the reflection). Thus, the eigenvalues of \( \left[\tau_{\mathbf{a}}\right]_{\mathcal{B}}^{\mathcal{B}} \) are \( 1 \) and \( -1 \).