The group $GL\_n(\mathbb{Z})$ acts properly and isometrically on the space of homothety classes of scalar products on $\mathbb{R}^n$. This is a Riemannian manifold of nonpositive sectional curvature.


Is there a similar space for the case of $GL\_n(F\_p[x])$. Maybe one can construct a building or something like this. I guess if the space I am looking for exists it should be rather well known. Otherwise I apologize for the vagueness of this question. 



On which space does $GL_n(F_p[X])$ act nicely?
The group $GL\_n(\mathbb{F}\_p[x])$ acts on the Bruhat-Tits building for $GL\_n$. The vertex set is $GL\_n(\mathbb{F}\_p((x^{-1})))/GL\_n(\mathbb{F}\_p[[x^{-1}]])$, and the higher simplices form sets of the form $GL\_n(\mathbb{F}\_p((x^{-1})))/I$ for various parahoric groups $I$. The action of $GL\_n(\mathbb{F}\_p((x^{-1})))$ on the left restricts to an action of $GL\_n(\mathbb{F}\_p[x])$.


When $n=2$ there is a nice exposition of the action in chapter 2 of Serre's *Trees* (as long as you're okay with $PGL\_2$ instead of $GL\_2$). You get an infinite sequence of 1-simplices that vaguely resembles the $SL\_2(\mathbb{Z})$-quotient of the complex upper half-plane. In higher rank, I don't think things can be quite so explicit.


You can also find actions of $GL\_n(\mathbb{F}\_p[x])$ on many other spaces, namely those that are defined over rings containing $\mathbb{F}\_p[x]$ and that come equipped with a natural algebraic or analytic action of $GL\_n$. For example, the analytification of the flag variety for $GL\_n$ over the completion of an algebraic closure of $\mathbb{F}\_p((x^{-1}))$ admits such an action.


