I understand that via the [Borel density theorem](http://www3.nd.edu/~andyp/notes/BorelDensity.pdf) given a finite dimensional (polynomial) representation of the simple non-compact Lie groups $SL\_n \mathbb R$ or $Sp\_n \mathbb R$, I get an irreducible representation when I restrict to $SL\_n \mathbb Z$ or $Sp\_n \mathbb Z$, respectively. 


I wonder if the restriction is injective on the set of isomorphism classes of irreducible polynomial representations. I.e. is it true that if $V$ and $W$ are irreducible polynomial $SL\_n \mathbb R$-representations and
$$Res\_{SL\_n \mathbb Z} V \cong Res\_{SL\_n \mathbb Z} W$$
then $V\cong W$?



Irreducible representations of $SL_n \mathbb Z$
(This is too long for a comment.) This follows from the fact that $\text{SL}\_n\mathbb{Z}$ is Zariski dense in $\text{SL}\_n\mathbb{R}$. Let $\phi:V\to W$ be a linear isomorphism such that for every $g\in \text{SL}\_n\mathbb{Z}$, the "conjugate" $\phi^g = g^{-1}\cdot\phi(g\cdot -)$ equals $\phi$. The subset of $\text{SL}\_n\mathbb{R}$ of all $g$ such that $\phi^g$ equals $\phi$ is the simultaneous zero locus of finitely many polynomial functions of the matrix entries of $g$: just look at the matrix entries of $\phi^g - \phi$ as functions of $g$. Thus, since these polynomials vanish on the Zariski dense subset $\text{SL}\_n\mathbb{Z}$, they vanish on all of $\text{SL}\_n\mathbb{R}$.


