ThmDex – An index of mathematical definitions, results, and conjectures.
Characteristic polynomial for a complex diagonal matrix
Formulation 0
Let $A \in \mathbb{C}^{N \times N}$ be a D6159: Complex square matrix such that
(i) $c_1, \ldots, c_N \in \mathbb{C}$ are each a D1207: Complex number
(ii) \begin{equation} A = \begin{bmatrix} c_1 & 0 & \cdots & 0 \\ 0 & c_2 & \vdots & \vdots \\ \vdots & \cdots & \ddots & \vdots \\ 0 & \cdots & \cdots & c_N \end{bmatrix} \end{equation}
Let $z \in \mathbb{C}$ be a D1207: Complex number.
Then \begin{equation} \text{Det}(z I_N - A) = \prod_{n = 1}^N (z - c_n) \end{equation}
Proofs
Proof 0
Let $A \in \mathbb{C}^{N \times N}$ be a D6159: Complex square matrix such that
(i) $c_1, \ldots, c_N \in \mathbb{C}$ are each a D1207: Complex number
(ii) \begin{equation} A = \begin{bmatrix} c_1 & 0 & \cdots & 0 \\ 0 & c_2 & \vdots & \vdots \\ \vdots & \cdots & \ddots & \vdots \\ 0 & \cdots & \cdots & c_N \end{bmatrix} \end{equation}
Let $z \in \mathbb{C}$ be a D1207: Complex number.
This result is a particular case of R5561: Characteristic polynomial for an upper triangular complex matrix. $\square$