Sunday, 2 February 2014

linear algebra - Finding eigenvectors to eigenvalues, and diagonalization



I just finished solving a problem on finding eigenvectors corresponding to eigenvalues, however, I'm not sure if it is correct. I was wondering if someone could check my work:



For the matrix $W =

\begin{bmatrix}
1 & 2 \\
3 & 2\\
\end{bmatrix}$, I must find the eigenvectors corresponding to the eigenvalues, as well as a diagonal matrix similar to W.



I was able to find that the eigenvalues were equal to $\lambda = 4, -1$. Then, I used the equation $(A - \lambda I)v = 0$ to solve for the vector.



When $\lambda = 4$, I set up the equation $\begin{bmatrix}
1 & 2 \\
3 & 2\\

\end{bmatrix} - \begin{bmatrix}
4 & 0 \\
0 & 4\\
\end{bmatrix}$ = $\begin{bmatrix}
-3 & 2 \\
3 & -2\\
\end{bmatrix}$, which gave me the eigenvector $\begin{bmatrix}
2\\
3\\
\end{bmatrix}$.




For $\lambda = -1$, I did the exact same procedure and received the eigenvector which gave me the eigenvector $\begin{bmatrix}
1\\
-1\\
\end{bmatrix}$.



Did I do this part correctly? How do I find a diagonal matrix similar to $W$?


Answer



we can use Row operations to obtain a diagonal matrix similar to W




W =
\begin{bmatrix}
1 & 2 \\
3 & 2\\
\end{bmatrix}
$r_1-r_2=R_1$
gives $$W =
\begin{bmatrix}
-2 & 0 \\
3 & 2\\

\end{bmatrix}$$
then $R_2=2r_2$ gives
W =
\begin{bmatrix}
-2 & 0 \\
6 & 4\\
\end{bmatrix}
now $R_2=r_2+3r_1$ gives
$W =
\begin{bmatrix}

-2 & 0 \\
0 & 4\\
\end{bmatrix}$
and $R_1=\frac{1}{2}r_1$
gives $W =
\begin{bmatrix}
-1 & 0 \\
0 & 4\\
\end{bmatrix}$
which is in diagonal form, as required, as you can see the diagonal entries are the eigenvalues you calculated



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