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The problem I'm struggling is the following:

Let $n$ be a positive integer and let $A=% \begin{pmatrix} B & C\\ C & B \end{pmatrix} \in\mathcal{M}_{2n}(\mathbb{R}_{+})$, where $B,C\in\mathcal{M}_{n} (\mathbb{R}_{+})$. I am interested in putting conditions on $B$ and $C$ such that the spectral radius of $A$ is less than $1$.

I think that the answer is that $B+C$ and $B-C$ have spectral radius less than $1$, but I'm not very familiar working with block matrices and I don't know how to prove it (I came up with this guess by working with the scalar case, when $B$ and $C$ are just nonnegative numbers).

I am also interested in computing the powers of $A$ in terms of the powers of $B$ and $C$, in the case when the spectral radius of $A$ is less than $1$ (is this similar to the case when $n=1$, or is there something fundamentally different?)

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    We find that the characteristic polynomial of $A$ is the product of the characteristic polynomials of $B-C$ and $B+C$.2012-07-14
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    Are $B$ and $C$ supposed to be symmetric? If not, then $A$ won't be symmetric.2012-07-14
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    @GeoffRobinson: $B$ and $C$ are not necessarily symmetric. I apologize for the confusion. I edited the title.2012-07-14

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