# Math Help - Positive definite matrices

1. ## Positive definite matrices

Hi,

I have this problem that I really can't solve.

For which s and t do A and B have all eigenvalues λ > 0 (therefore positive definite)?
$A=\begin{pmatrix}
s & -4 & -4 \\ -4 & s & -4 \\ -4 & -4 & s
\end{pmatrix}$
and $B=\begin{pmatrix}
t & 3 & 0 \\ 3 & t & 4 \\ 0 & 4 & t
\end{pmatrix}$

I tried to use the characteristic polynomial but I can't solve it since it's third degree. I also tried Cholesky decomposition for symmetric matrices, but the elements of the matrices are too complicated to be used in a useful way.

Could someone help me?

2. There is another characterization of positive definiteness:

$x^T A x>0$ for all column vectors $x$.

This might be a little easier to use.

3. Originally Posted by roninpro
There is another characterization of positive definiteness:

$x^T A x>0$ for all column vectors $x$.

This might be a little easier to use.
Thank you very much!
I'll try that out. Looks much easier than what I was using...