3
$\begingroup$

It is well-known that if $\widetilde M\to M$ is a Galois cover of a compact Riemannian manifold $M$ with deck-transformation group $G$, then the growth of $G$ equals the volume growth of $\widetilde M$ (in the pullback metric).

Question. Is the same true when $M$ is a finite volume complete Riemannian manifold and $G$ is finitely generated?

The usual proof (a la Svarc-Milnor) argues that $G$ and $\widetilde M$ are quasiisometric (since $M$ is compact), and then uses that growth is a quasiisometry invariant. One could hope that the reasoning extends to the finite volume case when quasi-isometry is replaced by measure equivalence (ME), but growth type is not an invariant of ME. On the other hand, I do not have counterexamples for the above question.

$\endgroup$
2
  • $\begingroup$ what do you mean by equal? is $\exp(2n)=\exp(3n)$? $\endgroup$
    – YCor
    May 19, 2013 at 14:16
  • $\begingroup$ By "equal" I mean that the growth functions have the same growth type, i.e. they dominate each other, where $g$ dominates $f$ if and only if $f(t)\le Ag(At+B)+B$ for all $t$ and some constants $A, B$. $\endgroup$ May 19, 2013 at 14:34

1 Answer 1

8
$\begingroup$

No, it's not true. A cusp is a counterexample (see below for an example with no boundary). Here by cusp I mean the subset $\{\text{Im}(z)\ge 1\}$ of the upper half-plane with hyperbolic metric modulo $z\mapsto z+1$. This has finite volume and the fundamental group has linear $\mathbf{Z}$ growth, but the universal covering, which is a horodisc, has exponential growth (because the $n$-ball contains, for some $c>0$, the rectangle $[0,e^{cn}]\times [1,2]$, which contains exponentially many disjoint balls $[m,m+1]\times [1,2]$, which all have the same volume).

If you want no boundary, take the cylinder $\mathbf{C}$ modulo $z\mapsto z+1$, endow it with the cusp Riemannian metric on both components of $\{|\text{Im}(z)|\ge 1\}$, and extend arbitrarily to a Riemannian metric in between.

$\endgroup$
1
  • $\begingroup$ This is very helpful. I see now that I have had some silly confusions about growth. $\endgroup$ May 19, 2013 at 14:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.