Find the eigenvalues and a basis for each eigenspace of matrix A:
\begin{bmatrix}
1 & -3 & 3 \\
2 & -2 & 2 \\
2 & 0 & 0 \\
\end{bmatrix}
I found the eigenvalues by computing $|A-\lambda I|$:
$\lambda_1 = 0,$
$\lambda_2 = 1,$
$\lambda_3 = -2$
How do I find a basis for each eigenspace of matrix A?
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I tried the following:
$\lambda = 0:$
\begin{bmatrix}
1 & -3 & 3 \\
2 & -2 & 2 \\
2 & 0 & 0 \\
\end{bmatrix}
Do reduced row echelon form:
\begin{bmatrix}
1 & 0 & 0 \\
0 & 1 & -1 \\
0 & 0 & 0 \\
\end{bmatrix}
Does this mean one of the results is
\begin{bmatrix}
0\\
t\\
t\\
\end{bmatrix}
and to find the other two answers, do the same thing except set $\lambda$ equal to the other two values? Or does finding the basis for each eigenspace of matrix A mean something different?
Best Answer
Correct, you subtract each eigenvalue from the main diagonal and use row reduction to find the eigenvectors. You may also want to spend a minute or two trying to spot the eigenvectors because that tends to be faster than row reduction. For instance in your case the second and third columns are scalar multiples of each other and you can use this information to get the eigenvector $(0,1,1)^T$, which corresponds to the eigenspace $\{(0,t,t)^T : t \in \mathbf{R} \}$.
For the eigenvalue of $1$ you are looking for a vector $v$ with $Av = v$. If $v = (a, b, c)^T$ then $Av = (a - 3b + 3c, 2a - 2b + 2c, 2a)^T$. Thus $2a = c$ and we can now do this again with $A(a,b,2a)^T = (7a - 3b, 6a - 2b, 2a)^T$. This gives you the equations $7a - 3b = a$ and $6a - 2b = b$, both equivalent to $6a - 3b = 0$. Hence one eigenvector ($a = 1$) is $(1,2,2)^T$. This process is basically row reduction. I am making use of the fact that the third row has just one entry to try to move through the steps a bit faster. For practice, use row reduction to verify that this is the correct eigenvector.