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13
votes
Accepted
Can you cover a genus a billion hyperbolic surface with 15 balls?
Your conjecture is false. Every nonorientable closed connected surface of negative Euler characteristic, admits a hyperbolic metric such that the surface is covered by 3 embedded disks. Hence, for eac …
12
votes
Is there an absolute geometry that underlies spherical, Euclidean and hyperbolic geometry?
The question is what to modify and how much one is willing to modify. As everybody who seriously thought about elementary geometry knows, Euclid's axioms are inadequate. There are several "standard" a …
9
votes
Accepted
Examples of the Thurston geometries with transitive Lie group action
This is an answer to questions 7 and 8 (I have to say, having 8 questions in one post is way too much for my taste):
Suppose that $M$ is a finite-volume quotient of $H^3$ or a compact quotient of $H^ …
8
votes
Accepted
Reference for shortest educational path to (Riemannian) hyperbolic plane
Try sections 1-15 of this paper:
Cannon, James W.; Floyd, William J.; Kenyon, Richard; Parry, Walter R., Hyperbolic geometry, Levy, Silvio (ed.), Flavors of geometry. Cambridge: Cambridge University P …
8
votes
Accepted
Optimal $\delta$ for Gromov's $\delta$-hyperbolicity of the hyperbolic plane
Indeed, the hyperbolic plane is $\log(2)$-hyperbolic (with the 4-point definition of hyperbolicity) and this is the optimal constant. The result is nontrivial and first appeared as Corollary 5.4 in
Ni …
8
votes
Accepted
Ergodicity of action of finite index subgroups in the boundary
Let $X$ be a Riemann surface of class $P_G$ (i.e. which carries a Green function) but is Liouville (i.e. admits no nonconstant bounded harmonic functions). One way to construct these is to take a $\m …
7
votes
Gromov hyperbolicity constant vs. Gromov-Hausdorff distance to a tree
Just to remove this question from the un-answered list. First of all, if $d_{GH}(X,Y)\le \epsilon$ then there is a $(1,2\epsilon)$-quasi-isometry $X\to Y$.
See for instance
Burago, D.; Burago, Yu.; Iv …
5
votes
Accepted
Examples of hyperbolic manifolds of dimension $\geq$ 3 with disjoint totally geodesic hypers...
I do not have a self-contained reference, but the key is
Long, D. D.; Reid, A. W., Constructing hyperbolic manifolds which bound geometrically, Math. Res. Lett. 8, No. 4, 443-455 (2001). ZBL0992.57023 …
5
votes
Accepted
Lengths of generators of surface group
In order to remove this question from the "unanswered list." Let $\epsilon>0$ be the Margulis constant for the hyperbolic plane (with curvature $-1$). Then for every complete hyperbolic surface $S$, i …
4
votes
Accepted
Hadamard submanifolds of $k$-fold product of hyperbolic plane
For the $i$th factor ${\mathbb H}^2$ in the product of hyperbolic planes, pick a complete geodesic $c_i$, $i=1,...,k$. The product
$$
F=c_1\times ... \times c_k\subset X=\prod_{i=1}^k {\mathbb H}^2
$ …
4
votes
Canonically representing the monodromy of a hyperbolic manifold fibered over $S^1$
Here is an answer of sorts; it is not completely canonical though. First of all, you have to pick a conformal or hyperbolic structure on the fiber $\Sigma$. This can be made almost canonical, since th …
2
votes
Can a hyperbolic three-manifold have 𝑛 toric boundary components?
As for conformal moduli of the tori (more precisely, Teichmuller parameters) that appear: It is hard to tell, afaik, there is no explicit description. We know that these will be elements of $\bar{{\ma …
2
votes
Geometry and topology of Fuchsian character varieties
Let $\Gamma< PSL(2,\mathbb R)$ (not $SL(2,\mathbb R$)!) be a cocompact Fuchsian group. Given a Lie group $G$ one defines
The representation variety $R(\Gamma, G)=Hom(\Gamma,G)$. One further defines $ …
1
vote
Hyperbolic 3-manifolds inside algebraic varieties
Let $S$ be a complete hyperbolic surface of finite area, $f: S\to S$ be a pseudo-Anosov homeomorphism which lies in a torsion-free finite index subgroup $\Gamma$ of the mapping class group $Mod_S$ (fo …
1
vote
Accepted
Inheritance of arithmeticity properties in orbifold strata
Here is what I think is the correct setup:
Let $X$ be a symmetric space of noncompact type, $\Gamma$ is a lattice in the isometry group of $X$. Then $\Gamma$ has finitely many $\Gamma$-conjugacy class …