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Feb 28 at 18:11 comment added YCor @ghc1997 Yes, it's true (nothing to do with $\mathbf{Z}$: it's true for $A\in\mathrm{GL}_n(\mathbf{Q})$). Just consider kernel and image of $(A^{k!}-1)^n$ for large enough $k$.
Feb 28 at 18:01 comment added ghc1997 Hi Yves, I have a slightly related question. Let $A\in\mathrm{GL}_n(\mathbf{Z})$, if we denote $V_{\mathrm{nru}} $ to be the sum of characteristic subspaces of $A$ relative to eigenvalues that are not roots of unity. Can we always find a basis of $V_{\mathrm{nru}} $ in $\mathbf{Q}^n $, i.e. do we have $\mathrm{dim} V_{\mathrm{nru}} =\mathrm{dim} V_{\mathrm{nru}} \cap\mathbf{Q}^n$?
Feb 16, 2016 at 15:09 vote accept Lee Mosher
Feb 16, 2016 at 7:43 history edited YCor CC BY-SA 3.0
Added additional argument
Feb 15, 2016 at 23:22 history answered YCor CC BY-SA 3.0