Given $A$, find invertible $B$ such that $B^{-1}AB$ is positive Given $A \in Mat(n,n,\mathbb R)$, is there always an invertible matrix B, such that $B^{-1}AB$ is positive, assuming all eigenvalues of A are positive and simple ?
If yes, is it possible to classify all invertible matrices B with this property ?
I tried hard to work with the perron-frobenius-theorem, but the problem is reversed.
 The perron-frobenius-theorem starts with a positive matrix and ensures at least one
 simple strictly positive eigenvalue. So, I cannot see how I can use it for my
 problem because I want to get a positive matrix starting with a non-positive one.
Additional question : Is there a special name or a notation for matrices with 
 strict positive entries ?
 A: The answer is positive. In dimension $n=1$ it's obvious, so from now $n\geq 2$. Let $\lambda_1<\lambda_2<\ldots<\lambda_n$ be the eigenvalues of $A$ , and let $X$ invertible such that $X^{-1}AX=diag(\lambda_1,\ldots,\lambda_n):=D$. Now consider the matrix
$$U=\begin{vmatrix}1&-x&0&\ldots &0\\
1&0&-x&\ldots &0\\
\vdots & 0 & 0 &\ldots &-x\\
1&1&1&\ldots &1
\end{vmatrix}$$
where $x\geq 1$. $U$ is invertible, and apart for a positive factor we have
$$U^{-1}=\begin{vmatrix}
x&x&x&\ldots&x^2\\
-x-k&1&\ldots&1&x\\
1&-x-k&\ldots&1&x\\
\vdots&\vdots&\ddots&\vdots&\vdots\\
1&1&\ldots&-x-k&x
\end{vmatrix}$$
where $k=n-2$. Let's show that, for a sufficient large $x$, $U^{-1}D\,U$ is positive (i will use $c$ to denote terms independent of x)
$$U^{-1}DU\begin{pmatrix}1\\0\\\vdots\\0\end{pmatrix}=\begin{pmatrix}\lambda_1x+\ldots+\lambda_{n-1}x+\lambda_nx^2\\x(\lambda_n-\lambda_1)+c\\x(\lambda_n-\lambda_2)+c\\\ldots\end{pmatrix}$$
$$U^{-1}DU\begin{pmatrix}0\\1\\\vdots\\0\\0\end{pmatrix}=\begin{pmatrix}x^2(\lambda_n-\lambda_1)\\x^2\lambda_1+x\lambda_n+c\\x(\lambda_n-\lambda_1)+c\\\vdots\\x(\lambda_n-\lambda_1)+c\end{pmatrix}$$
and so on. It's easy to see that, for sufficinet large $x$, all these vectors are positive. Now, set $B=XU$, then $B^{-1}AB$ is positive.
