# Thread: repeated eigenvalue general solution problem

1. ## repeated eigenvalue general solution problem

Hello all,

I am having trouble solving this repeated eigenvalue problem.

$
x'=\left|\begin{array}{ccc}0&1&1\\1&0&1\\1&1&0 \end{array} \right|{x}$

solving for the eigenvalues i get
$
\left|\begin{array}{ccc}-\lambda&1&1\\1&-\lambda&1\\1&1&-\lambda \end{array} \right|=-(\lambda+1)^2(\lambda-2)=0$

which means
$\lambda_1 = -1, \lambda_2 = 2$

solving for the non-repeated solution using the eigenvalue
$\lambda_1 = 2$
$
\left|\begin{array}{ccc}-2&1&1\\1&-2&1\\1&1&-2 \end{array} \right|
$

which gives me the eigenvector and solution
$
x_1(t)=\left|\begin{array}{ccc}1\\1\\1\end{array} \right| e^{2t}
$

however when I try to solve for the repeated value
$\lambda = -1$
i get the matrix
$
\left|\begin{array}{ccc}1&1&1\\1&1&1\\1&1&1 \end{array} \right|
$

I dont know how to find a LI eigenvector from this matrix. I have the solution from the back of my textbook, but even after just arbitrarily taking the same eigenvector that they chose, I cannot come up with the same solution. I believe the repeated solution should be of the form
$x^{(2)}(t) = u.te^\lambda_2t + w.e^\lambda_2t$
where u and w are vectors, and u is actually the eigenvector from the matrix acquired using $\lambda = -1$

I am stuck here, any help would be greatly appreciated.

Thanks,
Nick

2. The system comes down to the equation x+y+z = 0, from which you can get two linearly independent vectors, e.g. (1,0,-1) and (1,-1,0).