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?
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 $(1+2i,-1,-8i)^T$