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8 votes
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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 …
Moishe Kohan's user avatar
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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 …
Moishe Kohan's user avatar
  • 12.3k
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 …
Moishe Kohan's user avatar
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13 votes
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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 …
Moishe Kohan's user avatar
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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 …
Moishe Kohan's user avatar
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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 …
Moishe Kohan's user avatar
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8 votes
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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 …
Moishe Kohan's user avatar
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4 votes
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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 $ …
Moishe Kohan's user avatar
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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 …
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1 vote
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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 …
Moishe Kohan's user avatar
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5 votes
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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 …
Moishe Kohan's user avatar
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9 votes
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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^ …
Moishe Kohan's user avatar
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5 votes
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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 …
Moishe Kohan's user avatar
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0 votes
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Thurston's metric is bounded

I am not sure how well you know the Teichmuller theory, but the basic thing to understand is that $\mathcal T$ is not the space of hyperbolic metrics on $\Sigma$: It is the space of pairs $(\sigma, [\ …
Moishe Kohan's user avatar
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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 $ …
Moishe Kohan's user avatar
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