0
$\begingroup$

Consider a real but not symmetric matrix $A$. To test if the matrix has positive eigenvalues, I've learnt from this forum that a symmetric matrix will be given by $B=A+A^T$. If all the eigenvalues of $B$ are positive, then it follows that A also has all the eigenvalues positive. So this is a sufficient condition.

For example, consider

$$A=\begin{bmatrix}1&4\\0&1\end{bmatrix}$$

It so happens that $B$ has one negative eigenvalue. Whereas $A$ has both positive eigenvalues. So what is a necessary (and sufficient) condition that $A$ has all positive eigenvalues?

Looking at Gershgorin's theorem , further rises the possibility of complex eigenvalues.

References:

  1. Tests for positive definiteness for nonsymmetric matrices
  2. p.322,Linear Algebra and its Applications, Gilbert Strang.
  3. Necessary and sufficient condition for all the eigenvalues of a real matrix to be non-negative
$\endgroup$
7
  • 1
    $\begingroup$ A non-symmetric matrix may have complex eigenvalues. Do you want their real parts to be positive? $\endgroup$ Aug 1, 2017 at 17:20
  • 1
    $\begingroup$ A search for 'Lyapunov equation' might be helpful. $\endgroup$
    – ekkilop
    Aug 1, 2017 at 17:39
  • $\begingroup$ @RodrigodeAzevedo I want all the Eigen Values to be real $\endgroup$ Aug 2, 2017 at 7:06
  • $\begingroup$ @TilakMallikarjun That is why symmetry is nice. It does ensure that the eigenvalues are real. $\endgroup$ Aug 2, 2017 at 9:03
  • $\begingroup$ @RodrigodeAzevedo, I am having a non-symmetric matrix, whose sign of Eigen values determine the nature of a phenomena. So I want to know if at all it is possible with the methods in Linear algebra to get an idea of the same without actually solving for Eigen values. $\endgroup$ Aug 2, 2017 at 10:08

1 Answer 1

0
$\begingroup$

Your result is false: take $A=\begin{pmatrix}1&-1\\1&1\end{pmatrix}$ then $A+ A^T=2I_2$ has his eigenvalues positive whereas eigenvalues of $A$ are $1 \pm i$ not even real...

$\endgroup$
2

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .