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Section 8.2 Generalized Eigenspaces

Definition 8.2.1. Generalized Eigenspace.

Let \(A\) be an \(n \times n\) matrix and \(\lambda\) an eigenvalue of \(A\) with algebraic multiplicity \(m\text{.}\) The generalized eigenspace of \(A\) corresponding to \(\lambda\) is:
\begin{equation*} V(\lambda) = \{\mathbf{v} \in \mathbb{C}^n \mid (A - \lambda I)^m \mathbf{v} = \mathbf{0}, m\in \mathbb{N}_{> 1}\} \end{equation*}
Note that the value \(m\) is no more than the algebraic multiplicity of the eigenvalue \(\lambda\text{.}\)

Insight 8.2.3.

Understanding Generalized Eigenspaces: While regular eigenspaces consist of vectors \(\mathbf{v}\) such that \(A\mathbf{v} = \lambda\mathbf{v}\text{,}\) generalized eigenspaces include vectors that eventually become eigenvectors after applying \((A - \lambda I)\) enough times. These vectors form "chains" that give rise to the Jordan block structure.
Example: Find a basis for the generalized eigenspace \(V(2)\) of the matrix
\begin{equation*} A=\left(\begin{array}{rrrrr} -1 \amp 0 \amp 0 \amp 0 \amp -9 \\ 0 \amp 2 \amp 0 \amp 0 \amp 1 \\ 0 \amp 0 \amp 2 \amp 0 \amp 0 \\ 0 \amp 0 \amp 1 \amp 2 \amp 0 \\ 1 \amp 0 \amp 0 \amp 0 \amp 5 \end{array}\right) \end{equation*}
Exercise: Let \(A = \left(\begin{array}{rrrr} 2 \amp 1 \amp 0 \amp 0 \\ 0 \amp 2 \amp 1 \amp 0 \\ 0 \amp 0 \amp 3 \amp 0 \\ 0 \amp 1 \amp -1 \amp 3 \end{array}\right)\) Find all eigenvalues of \(A\) and their algebraic multiplicities. Then find a basis for each generalized eigenspace.