Complex eigenvalue

eigenvalues-eigenvectorslinear algebramatrices

Question: Suppose 𝐴 is a 3×3 matrix with real entries that has a complex eigenvalue −1+8𝑖 with corresponding eigenvector

\begin{bmatrix}
1-2i\\
-1\\
8i
\end{bmatrix}

Find another eigenvalue and eigenvector for 𝐴.

What should i do?

Best Answer

$A$ has real entries so $\bar{A}=A$. Now just conjugate $Av=\lambda v$ you get that $\bar{\lambda}$ is an eigenvalue corresponding to $\bar{v}$. (Remember this is only true because $A$ has real entries).

So $-1-8i$ is an eigenvalue with eigenvector $(1+2i,-1,-8i)^T$

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