# Eigenvectors and Eigenvalues

• Mar 22nd 2010, 07:10 AM
nicolem1051
Eigenvectors and Eigenvalues
It says: Show that v is an eigenvector of A and find the corresponding eigenvalue.

A =
[3 0 0
0 1 -2
1 0 1]

v =
[2
-1
1]
• Mar 22nd 2010, 08:55 AM
TheEmptySet
Quote:

Originally Posted by nicolem1051
It says: Show that v is an eigenvector of A and find the corresponding eigenvalue.

A =
[3 0 0
0 1 -2
1 0 1]

v =
[2
-1
1]

v is an eigen vector iff $\displaystyle Av=\lambda v$ where $\displaystyle \lambda$ is the eigenvalue for v.

$\displaystyle Av=\begin{bmatrix}6 \\ -3 \\ 3 \end{bmatrix} =3v$
• Mar 22nd 2010, 09:54 AM
nicolem1051
thank you so much, that's the answer I got but I don't understand why it is 3. For some reason I am not getting it(Doh)
• Mar 22nd 2010, 11:46 AM
canto88
to find the eigenvalues all u have to do is find the characteristic equation det(A-λI)=0

for example..the matrix A[ 3 0 0
0 1 -2
1 0 1]

u write it like this A[ 3-λ 0 0
0 1-λ -2
1 0 1-λ]
and then you expand by Column 2
and therefore u get (1-λ)(3-λ)(1-λ)=0
hence the eigenvalues are λ=1, λ=3, λ=1.

You can now find the corresponding eigenvectors u, v and w.
Let u=[a,b,c]. If Au=λu where λ=1 and 3 in this case. u solve the system Au=λu and u find the eigenvectors.