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Euclidean, hyperbolic, discrete, convex, coarse geometry, metric spaces, comparisons in Riemannian geometry, symmetric spaces.

4 votes
Accepted

Amalgmated free product of hyperbolic groups with one malnormal and one virtual factor is hy...

As Misha says in comments, as long as H is also quasiconvex in $G_1$ then you will be able to apply the combination theorem. One can deduce this directly from the 'usual' statement that an amalgam of …
HJRW's user avatar
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5 votes
Accepted

Angles and Busemann function in CAT(0)

The answer is "no" even in the hyperbolic plane $\mathbb{H}^2$. Consider the horocycle about $\xi$ through $x$: this is the set of points such that $b_{\xi}(z)=0$. Let $\gamma$ be the geodesic throug …
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8 votes
Accepted

QVH characterization of virtually special groups

Being virtually special is actually a group-theoretic property, independent of the cube complex (at least in the word-hyperbolic case of interest to Agol). More precisely, Haglund and Wise, in their …
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9 votes
Accepted

CAT(0)-groups in dimension 2

A very wide array of properties are compatible with these hypotheses. Burger--Mozes famously gave examples of infinite simple groups of this form. Earlier, Wise and Bhattacharjee had independently g …
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3 votes

Gluing hexagons to get a locally CAT(0) space

The fourth example was studied by Brady and Crisp in their CMH paper CAT(0) and CAT(-1) dimensions of torsion-free hyperbolic groups, so it would be reasonable to call its fundamental group the "Brady …
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17 votes

Applications of the notion of of Gromov-Hausdorff distance

Gromov's Theorem was, as far as I'm aware, the first but very far from the last application of Gromov–Hausdorff distance to group theory. One particularly fruitful line of reasoning starts with a seq …
13 votes
Accepted

Walls of CAT(0) cube complex sufficiently far apart implies intersection of stabilizers finite

Here's a proof. Lemma: Suppose $G$ is a (word-)hyperbolic group acting properly discontinously, cocompactly and faithfully on a C⁢A⁢T⁢(0) space $X$. Then there is a uniform bound $R_0$ on the width …
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5 votes

Convex hull in CAT(0)

It follows quickly from the definition that closed balls are convex. [Proof: Let p,q be in the ball of radius R about o, and let x lie on the geodesic from p to q. Then $d(o,x)\leq d(\bar{o},\bar{x} …
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3 votes

Geometric or topological results from group theory

I'm still a little uncertain about this question, but I'll try to say something about the Virtual Haken conjecture (discussed above) and in the process explain why I think it's a good example. The Vi …
9 votes

Amalgamated product acting on CAT(0) cube complex

To extend the gluing result from Bridson--Haefliger to non-positively curved cube complexes, it is important to work in the correct category. If we want the result to also be a non-positively curved c …
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