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Last time: **Theorem 21** (Groves–Manning–Osin): If is hyperbolic rel then there exists a finite subset such that if then

(a) is injective;

(b) is hyperbolic rel .

**Theorem 22** (Gromov, Olshanshkii, Delzant): If is hyperbolic relative to the infinite cyclic then there is a such that for all there exists a hyperbolic such that for each .

The proof is an easy application of Groves–Manning–Osin.

**Definition**: If (infinite cyclic) is malnormal then we say are **independent**. A group G is **omnipotent** if for every independent there exists a such that for all there exists a homomorphism $\phi$ from to a finite group such that for all .

Omnipotence strengthens residual finiteness for torsionfree groups.

**Exercise 29**: If every hyperbolic group is residually finite then every hyperbolic group is omnipotent.

We’ll finish off by talking about a similar theorem of Agol–Groves–Manning. I’m going to seem a little cavalier about torsion. This is OK. In fact, if every hyperbolic group is residually finite then every hyperbolic group is virtually torsionfree.

**Theorem 22** (Agol–Groves–Manning): If every hyperbolic group is residually finite then every quasi-convex subgroup of any hyperbolic group is separable.

Let . The idea is to Dehn fill to get a new hyperbolic group in which the image is finite and . If we could do this, we would be done by residual finiteness. This works if is malnormal. But it probably isn’t. Fortunately, we can quantify how far is from being malnormal:

**Definition**: The **height** of is the maximal such that there are distinct cosets such that the intersection

is infinite.

H is height iff is finite. In a torsionfree group, is height iff is malnormal.

**Theorem 23** (Gitik, Mitra, Rips, Sageev): A quasiconvex subgroup of a hyperbolic group has finite height.

Agol, Groves and Manning are able to prove:

**Theorem 24**: Let be a (torsionfree) residually finite hyperbolic group, and a quasiconvex subgroup of height . Let . Then is an epimorphism to a hyperbolic group such that

(i) is quasiconvex in ;

(ii) ;

(iii) has height .

The idea of the proof of Theorem 24 is to Dehn fill a finite index subgroup of a maximal infinite intersection of conjugates of . Theorem 22 is an easy consequence.

Recall, that for any graph we built a combinatorial horoball . For a group and a collection of subgroups and a generating set , we built the *augmented Cayley graph* by gluing copies of . is hyperbolic relative to if and only if is Gromov hyperbolic.

**Exercise 28:** If and are finitely generated, then is hyperbolic relative . (*Hint: is a graph of spaces with underlying graph a tree and the combinatorial horoballs for vertex spaces.*)

**Example:** Suppose is a complete hyperbolic manifold of finite volume. So, acts on . Let be a subset of consisting of points that are the unique fixed point of some element of . So acts on , and there only finitely many orbits. Let be stabilizers of representatives from these orbits and let . Then, is hyperbolic relative to .

**Example:** Let be a torsion-free word-hyperbolic group. Then, is clearly hyperbolic relative to . A collection of subgroups is *malnormal* if for any , implies that and . is hyperbolic relative to if and only if is malnormal.

The collection of subgroups is the collection of *peripheral subgroups*.

**Lemma 31:** If is torsion-free and hyperbolic relative to a set of quasiconvex subgroups , then is malnormal.

**Sketch of Proof:** Suppose that is infinite. Consider the following rectangles: Note that if , then is contained in a -neighborhood of . Now, there exists infinite sequences and such that . Look at the rectangles with vertices . The geodesics in between 1 and and and go arbitrarily deep into the combinatorial horoballs. Therefore, they are arbitrarily far apart. It follows that these rectangles cannot be uniformly slim.

Let where each . Write . Call this the *Dehn filling* of .

**Note:** If is hyperbolic relative to , then is hyperbolic.

**Theorem 21:** (Groves-Manning-Osin). Suppose is hyperbolic relative to . Then, there exists a finite set contained in such that whenever we have

- is injective for all , and
- is hyperbolic relative to the collection ;

In particular, if are all hyperbolic, then so is .

One application of this theorem is a simple proof of a theorem of Gromov, Olshanskii, and Delzant:

**Theorem 22:** Let be hyperbolic and suppose is malnormal, with each infinite. Then, there is constant such that for all positive integers there is an epimorphism to a hyperbolic group such that for each .

Some intuition: Recall that if is a closed hyperbolic manifold

then is word-hyperbolic. However, a lot of interesting hyperbolic manifolds are not closed.

**Example:** Let be the figure 8 knot.

Then the complement admits a complete hyperbolic metric and is of finite volume.

So, here we have an example of a hyperbolic manifold which is not compact but is of finite volume. This is almost as which is almost as natural as being closed.

is homotopy equivalent to , the complement of a thickened in .

is a compact manifold with boundary and its interior admits a hyperbolic metric. The boundary of is homeomorphic to a 2-torus, so induces a map . By Dehn’s lemma, the map is injective so cannot be word hyperbolic. The point is that acts nicely on but no cocompactly so the Svarc=Milnor lemma does not apply.

The torus boundary component of corresponds to a **cusp** of .

The point is that we can use cusped manifolds like to build a lot of manifolds and in particular a lot of hyperbolic manifolds.

Take and a solid Torus .

Choose a homeomorphism

**Definition:** The manifold is obtained from by **Dehn filling** .

We now want to understand what we have done to . The map induces a map :

The surjectivity of follows from the fact that is a homeomorphism. The Seifert Van Kampen theorem implies that , where denotes the normal closure of .

**Gromov-Thurston theorem: ** Let M be any compact hyperbolic manifold and be a component of homeomorphic to a 2-torus for all but finitely many choices of

the Dehn filling is hyperbolic.

Note: by finitely many we mean finitely many maps up to homotopy.

This is a very fruitful way of building hyperbolic manifolds. The next question to ask is whether we can do the same thing for groups. So, now we will try to develop a group theoretic version of this picture.

Let be a group theoretic graph with the induced length metric. Construct a new graph called the **combinatorial horoball** on as follows: Define the vertices . There are two sorts of edges in . We say that and are joined by a (horizontal) edge if and . We say that and are joined by a (vertical) edge for all .

For large enough and will have distance one and iff iff .

**Exercise 27**:

** (A).** For , .

** (B).** For any connected , is Gromov hyperbolic .

Let be a group and let be a finite set of finitely generated subgroups of . Choose a finite generating set for such that for each , generate . Then contains natural copies of .

Construct the **augmented Cayley graph** by gluing on combinatorial horoballs equivariantly.

where for each

and each /, is glued to along .

**Definition:** G is **hyperbolic rel ** if and only if is Gromov hyperbolic for some (any) choice of .

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