# Math Help - Another question about large powers of matrices

1. ## Another question about large powers of matrices

So the matrix given is...

$A=\left(\begin{array}{cc}1-p & p \\ p & 1-p\end{array}\right)$

...and I've established the eigenvectors are $\left(\begin{array}{c}1 \\ 1\end{array}\right)$ and $\left(\begin{array}{c}1 \\ -1\end{array}\right)$ with corresponding eigenvalues $1$ and $1-2p$. I'm then asked to show...

$A^n\left(\begin{array}{c}1 \\ 0\end{array}\right)=\frac{1}{2}\left(\begin{array} {c}1 \\ 1\end{array}\right)+\frac{1}{2}(1-2p)^n\left(\begin{array}{c}1 \\ -1\end{array}\right)$

...and I just can't seem to get it, the method I was taught seems to suggest I need to replace the $\left(\begin{array}{c}1 \\ 0\end{array}\right)$ with the eignevalues and eigenvectors multiplied togeter and then added together, but I still can't quite seem to get that expression.

2. Originally Posted by chella182
So the matrix given is...

$A=\left(\begin{array}{cc}1-p & p \\ p & 1-p\end{array}\right)$

...and I've established the eigenvectors are $\left(\begin{array}{c}1 \\ 1\end{array}\right)$ and $\left(\begin{array}{c}1 \\ -1\end{array}\right)$ with corresponding eigenvalues $1$ and $1-2p$. I'm then asked to show...

$A^n\left(\begin{array}{c}1 \\ 0\end{array}\right)=\frac{1}{2}\left(\begin{array} {c}1 \\ 1\end{array}\right)+\frac{1}{2}(1-2p)^n\left(\begin{array}{c}1 \\ -1\end{array}\right)$

...and I just can't seem to get it, the method I was taught seems to suggest I need to replace the $\left(\begin{array}{c}1 \\ 0\end{array}\right)$ with the eignevalues and eigenvectors multiplied togeter and then added together, but I still can't quite seem to get that expression.
$A^nX=\lambda^n X,A^n\left(\begin{array}{c}1 \\ 0\end{array}\right)=\frac{1}{2}(A^n\left(\begin{ar ray}{c}1 \\ 1\end{array}\right)+A^n\left(\begin{array}{c}1 \\ -1\end{array}\right))=\frac{1}{2}\lambda_1^n \left(\begin{array}{c}1 \\ 1\end{array}\right)+\frac{1}{2}\lambda_2^n\left(\b egin{array}{c}1 \\ -1\end{array}\right)$

3. Cheers, still not entirely sure why it's $\frac{1}{2}$, but I'm not sure that matters. Although I don't think the $\frac{1}{2}$ was used in the other example I have... oh well.

4. Originally Posted by chella182
Cheers, still not entirely sure why it's $\frac{1}{2}$, but I'm not sure that matters. Although I don't think the $\frac{1}{2}$ was used in the other example I have... oh well.
(1,0)=0.5((1,1)+(1,-1))
why not sure?

5. Because that's just repeating what was said and not explaining it, but s'ok, I figured it out in the exam anyway.

6. This makes two threads in a row in which you have asked for help, got a response which you did not understand and then insulted a person who tried to help you. Do us all a favor and stop posting here.

7. Originally Posted by chella182
Because that's just repeating what was said and not explaining it, but s'ok, I figured it out in the exam anyway.
well..may be it is not clear what i have said..sorry..
$A^nX=A^{n-1}AX=A^{n-1}\lambda X=\lambda A^{n-2}AX=\lambda^2 A^{n-2}X=...=\lambda^n X$
then you can substitude all the $\lambda$ to the value given in the exercise.