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This tag is used if a reference is needed in a paper or textbook on a specific result.

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Local quasiconvexity in graphs of free groups with cyclic edge groups

You're right that hyperbolic graphs of free groups with cyclic edge groups are locally quasiconvex. This can be proved by combining subgroup separability with results about combination of quasiconvex …
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8 votes

The free group $F_2$ has index 12 in SL(2,$\mathbb{Z}$)

I'm late to the party, but here's how I think about this fact. The approach I will present is perhaps slightly higher tech than some, but has the advantage that it's a completely general way of comput …
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12 votes

Relative/acylindrical hyperbolicity of free-by-cyclic groups

This was proved by Jack Button and Robert Kropholler: arXiv link, see p.27. (Added: But there's a caveat; see the second update below.) J.O. Button, R. Kropholler Nonhyperbolic free-by-cyclic and on …
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4 votes
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Is the conjugacy problem solvable in $Out(F_n)$?

One result in this direction is given by Dahmani . His algorithm will determine conjugacy for pairs of atoroidal outer automorphisms, ie automorphisms that do not fix a non-trivial conjugacy class. …
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11 votes

Torsion-free virtually free-by-cyclic groups

Many more examples, including ones where the free kernel is finitely generated, arise by looking at knot complements. Let $K$ be any non-trivial knot with Alexander polynomial $\Delta_K(t)=1$, (appare …
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7 votes
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Passing to normal forms in graphs of groups

In comments, the OP indicates that what they really want is a uniqueness result for reduced words in arbitrary graphs of groups. (Indeed, what the question actually asks for, that any word can be tran …
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24 votes
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Modern references on hyperbolic groups

I think this is a great question, as there is still a need for an authoritative reference about (word-)hyperbolic groups. Since the textbook doesn't exist, I'd like to take the question in a slightly …
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11 votes
4 answers
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Examples of acylindrical 3-manifolds

Let $C$ be the compact cylinder $S^1\times [0,1]$. A 3-manifold $M$ with incompressible boundary is called acylindrical if every map $(C,\partial C)\to (M,\partial M)$ that sends the components of $\ …
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3 votes

Are braid groups conjugacy separable?

(26 April 2016: Updated to give a fuller answer.) I'm fairly confident this question is still open. As Ian Agol points out in comments, the 3-strand braid group is, by a happy accident, also the fun …
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12 votes
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A hyperbolic group with a small profinite completion

It's a famous open question whether every word-hyperbolic group is residually finite. Kapovich--Wise showed that this is equivalent to asking whether every non-trivial word-hyperbolic group has non-t …
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6 votes

What are some good group theory references?

For infinite discrete groups: Lyndon & Schupp is authoritative for classical, combinatorial methods. Bridson & Haefliger has a lot of material for more geometric classes, like hyperbolic and CAT(0) …
8 votes
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For which planar topological spaces $Z$ does there exist a hyperbolic group $\Gamma$ with $\...

There are many further examples with local cut points, which can be obtained by amalgams over $\mathbb{Z}$, as @YCor suggests in his comment. Perhaps the easiest example is obtained by gluing three on …
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3 votes

Existence of properly discontinuous and cocompact action

The question is phrased very generally, so I'm not sure if the following is what you're looking for. However, my answer to Does any surface of constant curvature admit a cocompact group action? might …
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15 votes
2 answers
863 views

A space of ideals

Definition: Let $R$ be a commutative ring with 1. Endow the power set $2^R$ with the product topology. The ideal space $\mathcal{I}(R)$ is defined to be subset of $2^R$ consisting of ideals, equippe …
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19 votes
0 answers
570 views

The oriented homeomorphism problem for Haken 3-manifolds

Haken famously described an algorithm to solve the homeomorphism problem for the 3-manifolds that bear his name (fleshed out by many others, including Hemion and Matveev who fixed some gaps). But it' …
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