largest eigenvalue of a symmetric matrix I have a matrix of the form:
$X=\Delta \Delta^T (\Phi+\Phi^T) P + P (\Phi+\Phi^T) \Delta \Delta^T  $,
where $\Delta$ is $N\times 1$ real, $P=P^T$. I know that such matrix is rank two, but after doing some simulations, i found that the largest eigenvalue is always positive. Is this a coincidence or are there any conditions that can ensure $\lambda_{max}(X)>0$? Thanks.
 A: Consider the following matrix
$a b^T + b a^T$
where $a=\Delta$, $b = P(\Phi+\Phi^T)\Delta$. Both $a$ and $b$ are N by 1 matrix.
Consider a vector $v = a + x b$ where $x$ is a real number. we have 
$ (a b^T + b a^T )v = [(a b^T + b a^T )a] + x[(a b^T + b a^T )b] $
$ =[a (b^T a) + b (a^T a)] + x[a (b^T b) + b (a^T b)] $
$ = [ (b^T a) + x (b^T b) ] a + [ (a^T a)+ x(a^T b)]b $
$ =(b^T a +x b^T b) a + (a^T a+ x b^T a) b$
Note that $(b^T a) = (a^T b) $ , $a^T a$, and $(b^T b)$ are all real numbers. The idea to construct $v$ this way is because the column space of $X$ is spanned from $a,b$, so the eigenvector must be the linear combination of $a$ and $b$.
Let
$ (b^T a +x b^T b) = \lambda  $
and
$ (a^T a+ x b^T a) = \lambda x  $
We have 
$(a^T a+ x b^T a) = x (b^T a +x b^T b) $
$ x^2(b^T b) - a^T a = 0 $
So we have the two roots of $x= \pm \sqrt{\frac{a^T a}{b^T b}}$
So
$\lambda = b^T a \pm \sqrt{a^T a \times b^T b}$ and by cauchy schwarz
$\lambda_{max} = b^T a + \sqrt{a^T a \times b^T b} > 0$
$\lambda_{min} = b^T a - \sqrt{a^T a \times b^T b} < 0$
So the conclusion is that the two nonzero eigenvalues has the opposite sign.
