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if and only if $G$ is connected and regular.

To prove the "only if", assume $J$ is a polynomial in $A$. Then $JA=AJ$. The entries in the $i$th row of $AJ$ are all equal to the sum of the entries in the $i$th row of $A$. The entries in the $i$th column of $JA$ are all equal to the sum of the entries in the $i$th column of $A$. Therefore, the sums of entries in a row of $A$ is constant. This shows that $G$ is regular. Also $G$ must be connected, for otherwise $i$ and $j$ is not connected and the $ij$th entry of $A^n$ is always zero and $J$ cannot be a polynomial in $A$.

But how can I show the other implication?

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    Let $B$ be a random real $n\times n$ matrix and $K=\sum_k c_k B^k$ be a polynomial, then $BK=KB$, because $[B^k,B]=0$. But $B$ is not regular. Where is the difference to your setup?2012-11-28

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