3
$\begingroup$

$\text{SL}(d,\mathbb R)/\text{SL}(d,\mathbb Z)$ has two interesting properties: on one hand it is non-compact, but on the other hand it admits a unique $\text{SL}(d,\mathbb R)$-invariant finite measure on $\text{SL}(d,\mathbb R)/\text{SL}(d,\mathbb Z)$.

I wonder if there exists a bounded measurable subset $F$ of $\text{SL}(d,\mathbb R)$ (with subspace topology, of course) that is Borel isomorphic to $\text{SL}(d,\mathbb R)/\text{SL}(d,\mathbb Z)$?

By "Borel isomorphic" I mean: There exists a bijection $f:F\subset \text{SL}(d,\mathbb R) \to \text{SL}(d,\mathbb R)/\text{SL}(d,\mathbb Z)$ such that both $f$ and $f^{-1}$ are Borel measurable, with $F$ inheriting the subspace topology from $\text{SL}(d,\mathbb R)$. Of course, "up to a subset of measure zero" is always allowed.

If this is true, then it is good to know a constructive proof but the proof for the existence of such an $F$ will also be greatly appreciated!


Here I am not assuming $F$ is a "fundamental domain" for $\text{SL}(d,\mathbb R)/\text{SL}(d,\mathbb Z)$, whose definition itself has many interpretations and I asked $F$ to be bounded and thus may not be homeomorphic to $\text{SL}(d,\mathbb R)/\text{SL}(d,\mathbb Z)$ (but as a possible approach one may want to construct such an $F$ whose closure is homeomorphic to the one-point compactification of $\text{SL}(d,\mathbb R)/\text{SL}(d,\mathbb Z)$ modulo a set of measure zero). I only want this $F$ to be measure theoretically isomorphic to $\text{SL}(d,\mathbb R)/\text{SL}(d,\mathbb Z)$ but not topologically. Disproving this claim seems also very hard.

$\endgroup$
7
  • 4
    $\begingroup$ Existence of $F$ is clear, because any non-discrete Polish space is Borel isomorphic to $\mathbb{R}$. $\endgroup$
    – GH from MO
    Commented Jun 14, 2023 at 17:25
  • 1
    $\begingroup$ There is no Haar measure on $\text{SL}(d,\mathbb R)/\text{SL}(d,\mathbb Z)$ as it is not a group. $\endgroup$
    – GH from MO
    Commented Jun 14, 2023 at 18:02
  • 1
    $\begingroup$ @GHfromMO Sorry I meant $\text{SL}(d,\mathbb R)$-invariant measure on $\text{SL}(d,\mathbb R)/\text{SL}(d,\mathbb Z)$. Then it exists. $\endgroup$
    – user506835
    Commented Jun 14, 2023 at 18:03
  • 4
    $\begingroup$ Yes, you can make that happen. See my answer, and note that the measure of the $F$ constructed there can be chosen to be an arbitrary positive number. $\endgroup$
    – GH from MO
    Commented Jun 14, 2023 at 18:05
  • 1
    $\begingroup$ @GHfromMO Sorry I haven't logged in for a while.. just did it $\endgroup$
    – user506835
    Commented Jun 23, 2023 at 19:40

1 Answer 1

3
$\begingroup$

By the Iwasawa decomposition, $\text{SL}(d,\mathbb R)$ is homeomorphic to $\mathbb{R}^n\times\text{SO}(d,\mathbb R)$, where $n=(d^2+d-2)/2$. As $\mathbb{R}^n$ is homeomorphic to any open ball in $\mathbb{R}^n$, it follows that $\text{SL}(d,\mathbb R)$ is homeomorphic to a bounded Borel subset of $\text{SL}(d,\mathbb R)$; this homeomorphism can be constructed explicitly. Now take a nice fundamental domain of $\text{SL}(d,\mathbb R)/\text{SL}(d,\mathbb Z)$; e.g. take a suitable Borel subset. We see that this fundamental domain is also homeomorphic to a bounded Borel subset of $\text{SL}(d,\mathbb R)$, and we are done.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .